No, this is a very specific type of sin for a priest (along with “giving absolution” to his co-participants) which I think may be one of those that can only be pardoned by the Pope. It’s a very grave sin.
Absolving of an accomplice is of this sort and clearly raised. This term is also subject to no ambiguity. Sacrilege and derived words have a range of meanings, which does certainly include black Mass, but also include other things. I doubt that the letter is using language to in itself serve as a list of canonical charges.
If a priest breaks the sixth commandment, attempts to absolve the accomplice (and has the accomplice attempt to absolve him!) and then proceeds to celebrate Mass, that is quite sacrilegious enough in my books, though I do not know if it rises to the level of a chargeable offence under canon law.