What if the new Spidey was white but still not an established friend/ally/confidant? Would you still be on /. arguing that Marvel "pulled him out of the ether" for the sake of being "politically correct"?
I think it depends on the friends/allies/confidants/enemies in the comic so far. If the Ultimate Spiderman line is chock full of blacks, whites, latinos, asians, bis, straights, gays, trans, etc. then for a new character to appear that happens to have one or more of those characteristics is not particularly worth commenting about.
However, if the Ultimate Spiderman line is centered in a black neighborhood where Spiderman is black and the vast majority of his friends and enemies are also black, then yes, it would feel like Marvel "pulled him out of the ether" to make some sort of statement.
I haven't read the Ultimate Spiderman line so I can't really say either way. Maybe in that line MJ is asian, J. Jonah Jamison is black, Dr. Ock is hispanic, Electro is white, black cat is bi, Eddie Brock is gay, etc. In that universe it wouldn't be particularly PC for the new spiderman to be a new character that happens to be half-black, half-latino, maybe gay. Honestly, it wouldn't surprise me that much given other comics I've read from the Ultimate universe.