Why? Was Joss involved in it?
Remember, canon isn't a question of quality, but of creator involvement/endorsement.
Brian Lynch was handpicked by Joss.
Armstrong admits she was NOT chosen by Joss.
Brian & Joss talked about what Joss wanted S6 to accomplish and what he wanted the comic to accomplish.
Armstrong's never even spoken to Joss.
Joss named Brian the co-Producer.
No one can even verify that Joss has read Armstrong's work, just that Mutant Enemy "approves" of it.
Joss mapped out
After the Fall with Brian, and said he really liked Brian's ideas.
Armstrong violated canon in her first issue, contradicting what Joss, Fury & Bell say is the PRIMARY premise of Angel.
So you're right that it may be questionable. However, Brian mapped out
After the Fall with Joss and knows what he wants to happen to Angel & co. So even if Joss didn't pen issue #23-25, it's being written by a guy who Joss picked and who knows the overall plan for these characters.
---------- Post added at 05:36 PM ---------- Previous post was at 05:22 PM ----------
As far as I'm concerned Brian Lynch was named EP of
After the Fall (which he was) by Whedon, the same as Marti Noxon was named EP of Buffy S6 by Whedon and David Fury was of Angel S5. So whatever Marti wrote for S6 was canon, whatever Fury wrote for S5 is canon, and whatever Brian writes for AtF is canon.