It's hard to judge without details, but this is perhaps a question of metatheory. (This is what item 2 sounds like to me, but this is just an impression.)
It is not a requirement in mathematical papers to carefully explain aspects of the metatheory, but it is not prohibited to do so. In your case, it seems that their result is trivial in your metatheory while yours isn't. By clarifying your metatheory, you can carefully explain that in your paper. In many cases, you really don't need to say much about their result since the issue is likely to be with their assumptions or methodology, not with their conclusions. Such a critique should sound much tamer than an attack on their result.
Clarifying your metatheory will also separate your result from theirs in other ways. It is possible that their metatheory (presumably not carefully explained in their paper) does not see their result as trivial. Maybe it even sees your result as trivial! A reader that shares their point of view and not yours will immediately see the difference when reading your paper and will be able to appropriately interpret your work regardless of their personal stance. This could avoid further confusion of the same kind.
PS: Being a logician, I prefer the term "metatheory" for what many other mathematicians would call "context," "framework," "perspective," "methodology," or whatever. Don't read too much technicality into the term, the metatheory is always informal. (Only logicians occasionally need to make the metatheory formal in order to formally prove metatheorems.) If this is indeed the kind of scenario you're looking at, it should be relatively clear to you exactly what and how much you need to clarify about your own metatheory.