It does make sense, you have to take the prior sentances as well.
We wanted to make and aggressive playmaker, someone who doesn't just save a drowning ally, but who forces his enemies to drown themselves ... By his own hands.
It should have been worded better, as it seems like they really wanted to tease the water aspect of his kit, however it really wouldn't make any grammatical sense for 'their' to change meaning in a sentence.
In this sentence it's just simply poorly constructed. The first their is clearly pyke, however the second their could either mean pyke, or themselves. However it appears that 'themselves' is simply to emphasize the difference of who is doing the drowning. It's implied that the enemy is drowning his (pykes) ally and instead of just rescuing them he causes 'themselves' (the ones doing the drowning) do drown instead.
It probably should have read like so,
But who forces his enemies to drown instead .. by his own hands.
Themselves, no matter whom the second 'their' is refering too, is simply awkward in this sentence.