From what I've read on other places people just seem to be using the 'biphobic' bit to look like they're in the right for being mad that they can't court him with a self insert.
Some people may use the term in that way. Most of the context that I have seen it used in (whether it be here, tumblr, or other places) is referring to characters like the DA2 companions as anything other than bisexual constitutes an erasure of bisexuality. That someone isn't bisexual if they are only seen as in a relationship with a particular gender (which isn't true. Bisexuals can be monogamous).
In the case of the tweet, it seems like the term was applied in a similar way. If someone already has the assumption that we're some level of biphobic, it seems as though they read David's tweet as being "fully gay" means "no longer the inferior bisexual."
A few others have expressed their reservations (and some have expressed their disagreement and support too) on these forums too.