You have an interesting point, and you are right to notice the similarities. However, all the two pieces have in common are a chord progression; the actual melodies are not the same. Hence, I'm inclined to belived this is not a rip-off, as the chord progression used in both pieces is actually extremely common in film music, particularly in Middle-eastern themed movies. Both Horner and Silvestri, as well as countless other film composers have used this progression in past scores. David Arnold's Stargate uses this chord progression, as well as Horner's Glory, both of which predate Tomb Raider (which also borrows this chord progression from the Mummy Returns). To say that Horner's is a rip-off (at least of someone else) and Silvestri's is original is inaccurate, and, I think, unfair.
Now I could be wrong, but I don't think so here. Tell me what you think.
Take it easy.
(By the way, the clip from Tomb Raider sounded pretty interesting... is the rest of the score as good... I have not heard the whole thing)