So I was reading up on the Eleventh Hour adventure, and one of the pre-rolled PCs have the talent called "Athletic Fighter", which seems to be the Athletics version of Assassin strike. Is this legal, and is it in the core book?

If not, what tier would it be, and what aptitudes would it be aligned with?

There are a number of problems with the weapon specialist pre-gen; including the mentioned talent he has a higher movement rate then a character with an Agility bonus of 3, and there is no such thing as a handheld combat shotgun (which somehow is classed as a pistol); It could make sense if they had the compact or micro weapon upgrades (which are in Rogue Trader) but even these do not account for the +2 pen.

In reality its to balance the pre-gens but really the whole campaign is pretty skrewy.