Maybe this is why: According to MGMAT SC
guide, "the Present Perfect indicates either continued action or continued effect of a completed action."
Completed action = equipped to face any obstacle the environment could put in their path
I think the key lies in "indicates that an inability to adapt." So there still exists an effect. Even though what caused this current effect is their "sudden disappearance." Because "equipped to face" works in conjunction with "sudden disappearance" (with the "but") to make a point, and that point still holds today.
Also, if nothing else, we really have no other choice but to use the present perfect. (Let’s even pretend the “as” is not there because “appear as” is an incorrect idiom.) We know we can't use simple past because of the use of the simple past ("appear") before it (which we concede we need for parallelism) because "appear equipped" means you are observing that right now. "Jack appears equipped to fight." And based on what you said about them disappearing, one might consider Past Perfect if it were a choice here. Even if it was, "In general, you should use Past Perfect only to clarify or emphasize a sequence of past events. The earlier event should somehow have a bearing on the context of the later event. Moreover, if the sequence is already obvious, we often do not need the Past Perfect." And "Clauses linked by 'and' and 'but' do not require the Past Perfect unnecessary."
But like you, I hope an expert chimes in.