Attitudes that undermine the humanity of transgender individuals, however severe, regardless of intent.
This concept applies to racism, misogyny, and homophobia too by the way
When it comes to phrases like this that are common and not specialized (everyone should know what transphobia means), we need to be using language similar to that which the general public would accept. I am not so sure that if you walked around and asked people what transphobia was, that you would ever get this answer for the definition. I think the general public would say that it involves intention at the minimum, the severity required would likely be contested depending on the individual.
These more nuanced versions of terms need to be constrained in use. The gender studies definitions have their place but not in common discourse, especially not on a video game forum.