I have a colleague that went through the same thing too. Worked for IBM, jumped ship for another company, company was acquired by IBM, rinse, repeat. Just twice so far though.

In all reality, what would happen if the source code got leaked? Certainly Riot Games would be able to sue anyone who uses (or was suspected to have used) that proprietary code in a product. And if a company was dumb enough to use it, I can't imagine the final product would even be a blip on our radar.