Fair question.

It has possible answers, though. If there aren't three clerics you can't celebrate solemn Mass at all: analogously, if this was truly reserved to the clergy perhaps it would have been omitted. The Absolutio can be not facienda, according to the rubric.

The rubric I’m looking at says cantore incipiente and clerus cantat, which also strikes me as odd since I can't think of any other situation where a cantor intones and the clergy continue.

The Absolutio is not done with holy water and the cross unless a corpus is present, I believe. To me that suggests (and so does the name absolutio) that a priestly action was understood here, at least long in the past.

Just asking about practice and historical practice.

No, that's not correct. The absolution may be done after every Requiem Mass, whether the body be physically present, present morally, i.e. if the body had to be buried hastily or could not be brought to the church, which is what Stercky defines as force majeure, due to contagion or the orders of the civil authority (Stercky, Manuel de liturgie et cérémonial selon le rit romain t. I, nº 709, p. 683), or not at all, as would be the case for most Requiem Masses. In the first two cases, the special arrangements for the feet of the priest are followed, and the prayer Non intres is said; otherwise, one starts the responsory when the priest is ready, whether standing at the catafalque or over the cloth.

The absolution is so important that it can be done even after low Mass, even sung ad libitum, or on occasions when a Requiem Mass is not said, usually due to the impossibilities of the calendar, although it must be a clearly distinct ceremony from a Mass which is incompatible with the absolution, such as a Mass of a feast which is at the rank of double of the first class. (Stercky gets into all of those details, such as those required by a foundation to pray for the dead, c.f. t. 1, nº 701, p. 675).

The absolution is required after funerals, even if the absolution was already celebrated at some other point, and including after the Office, if the Mass was not sung, or the Subvenite, if even that is impeded, but the absolution is not required after other Masses, though Stercky implies that one does sing the absolution after sung or solemn Mass on All Souls (t. II, nº 452, p. 403).

If one celebrates the absolution, then the aspersion and incensing are required after sung Masses with or without ministers (S.R.C. March 5, 1904, Ceneten. ad. I, which is referred to by Stercky, t. I, nº 701, note 2, p. 676); I suppose that the ceremonies may be reduced after a low Mass, but the uploader of the Stercky volumes to the Internet Archive rather annoyingly put the pages four at a time instead of two, and somehow he still scanned two pages twice. In any case, if Mass is sung, you can't then proceed to only the Libera me; you must sing everything.

The S.R.C. had the following to say about it on the anniversary of death: Non ex obligatione sed ad arbitrium facienda est absolutio in anniversariis mortuorum. (S.R.C., July 31, 1665, nº 1322, ad VI, as cited in note 1, p. 135, in Fortescue, Ceremonies of the Roman Rite Described) But the point about the ease with which it may be celebrated stands, though the placement in the liturgical books is a little bit frustrating, as you'd think that the rite would follow the Mass as it normally does, but in the Liber Usualis before the Office of the Dead entirely.

It seems that in the case of the actual celebration of the absolution, the clergy are present around the coffin or catafalque, not simply in choir, if this is possible; Fortescue even includes them in his diagram of absolution following a pontifical Mass, so it's possible that this rubric pertains to this above all. Stercky seems to use "chœur" and "clergé" interchangeably, because the assumption is that solemn Mass is sung in a collegiate church, and one does things with the necessary changes if in a parochial church. I imagine that some haphazard memory of this is how the practice of priests surrounding the coffin of deceased priests came to be common.

All of this having been said, it doesn't tells us much about the practice before Leo XIII, but I'm convinced that the more just thing is to never omit the absolution, at least after a sung liturgy, even though on paper, the rite is ad libitum except for funerals, the principal Mass of All Souls, and whenever a foundation would require it.

cantore incipiente and clerus cantat

This likely presupposes the ideal situation when the cantor, too, is part of the clerus, so that this just means that it is not necessarily the celebrating priest who should intone. Preconciliar Roman ritual says instead "cantore incipiente circumstantes cantant". In the OF context I have always seen that it is the celebrant who intones, and the parish choir continues.