The video is of a church... the audio is from a ballpark?

It's pretty clear that all those people leaving are (at least subconsciously) aware that this hymn is not part of the liturgy.

Excuse my rant:
This kind of boring, disconnected garbage is what the (so-called) "Contemporary" liturgical movement assumes it fights against. In all the "Spirit of Vatican II" liturgy and music-planning workshops I've ever attended (which is a lot, mind you) I've never heard anyone suggest that we throw out chant and polyphony in favor of goofy folk songs. I only ever heard about replacing boring music and generically-devotional hymn texts with well played contemporary music and texts actually related to the scripture lessons and liturgical actions.

I'd prefer chant, and I've come to love traditional hymnody. But if my two options were that video or a Gather Comprehensive guitar-and-piano Mass, I know where I'd be. And I'm not the only one, I'm sure.

And this is not just ranting for it's own sake: Many priests and musicians believe that, even in theory, there are only those two options. For many regular PIPs, the local parish scene is such that those really are the only two options.

It gets worse. Promoters and purveyors of real liturgical music often group themselves (or are grouped in with, by their detractors) that older, supposedly-traditional musical paradigm.

We need to understand the anti-hermeneutic of continuity at work here...
Traditionalists tend to view the rise of "contemporary music" through a hermeneutic of rupture- that somehow adding guitars and pianos, or singing some different songs, was a major break with the prior praxis. It wasn't- it was its natural development, and (often) was an improvement.

The music being played in the video above is also not a rupture with the crappy contemporary goings-on that came before. It is a natural development, retaining the essence of what was there previously, but molded by the current trends toward traditionalism (without truly being traditional), in the same way that the crappiness of pre-V2 provincial liturgy retained its essence, but was molded by the then-current trend of modernism (without ever truly becoming modern).

Moving from non-liturgical music that people like or are familiar with to non-liturgical music people don't like and aren't familiar is not an improvement. It's like switching from donuts to raisin bagels and pretending that breakfast is now healthy (if I don't like it, it must be good for me!).

By the time that lady got through verse 1, I was done with verses 2 and 3.

Nice voice.
I don't ever care about PIPs leaving at any point passed "Deo gratias," it's over. We're not the Liturgy Police, we're there to sing for our pleasure through God's gift of voice and song.
The only thing that riles me is if there's an organ postlude and even one of the choir chats during it, they get the stink eye and shrivel before its lasar beam!

That is an understandable sentiment, PGA, and here's what I've thought about it for at least 20 years- When those PIPs stream (and scream while) out the doors, the sound eminating from within to outside those doors verily says: THIS IS A CHURCH! I'm not worried about whether passersby or PIPs assess a value to that reality, but we're not going anywhere, this is A CHURCH.
It's kind of the opposite of when you're driving peacefully, and some boom car pulls up bombing bass notes that rattle your teeth and offend your ears with obscenities.

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favorite recessionals: when A Marian hymn was sung the celbrant processed to the nearby statue of Mary while one complete verse of the hymn was sung. Another is quiet insturmental music that promotes a tone of pray and thanksgiving.

Well, at my NO parish, most people stampede for the door after the priest recesses, some hang about and chatter like it is Friday night at the mall, but at least some stick around to applaud! "Thank you! I'm here every Sunday. Be sure to tip your ushers!" [/tongue-in-cheek]

In the chapel I serve the priest goes out during the postlude, when the recessional hymn has already been sung. In the last years the only exception was the Sunday after Candlemas (unless it falls on a Sunday), when he would process out during the first verse to vest in alb and red stole instead of white or green chasuble, for the St Blase Benediction. (Per intercessionem sancti Blasii episcopi et martyris liberet te Deus ab omni malo gutturis et a quolibet alio malo. In nomine Patris et Filii et Spiritus sancti. Amen. I don't know if it is that popular in America as it is in Germany.)

At my parish, I'll get with the priest before Mass to let him know how many verses of the closing hymn will be sung. He'll stick around until the last verse. Including the pastor, we have 5 regular weekend celebrants. Thankfully, all of them cooperate. The PIPs generally stay until the priest leaves.

We sing the seasonal antiphon for the Blessed Virgin, followed by an instrumental postlude. If we're singing the tonus simplex, the celebrant & ministers stand facing the altar and singing the antiphon, after which they leave the sanctuary and process down the center aisle. If we use a polyphonic or larger accompanied setting, after standing for a short time facing the altar, they will then turn and process.