No it is not a fallacy to use abbreviations correctly. I don't think anyone said it was.

It can be a failure of communication to use even correct abbreviations the audience is not familiar with. And invoking the Mind Projection Fallacy, implicitly pointing out that the ignorance is in the mind of the reader who doesn't get it, misses the point that the communication failure lies in the failure to account for the probable ignorance of some portion of the audience, and communicate the point anyways by using full English words, which is rightly assigned to the commenter that used the abbreviation.

Of course, I understand that you did not get all that from the phrase "deliberately obscure", which itself is the sort of vague accusation I think we should avoid. (And I also doubt the obscurity was deliberate.)