20 years ago, if someone asked to raise a quarter of a billion into a video game, they'd be laughed out of the building. Now it happens often. Over the years, increasingly big-budget games got funded and created.
So, all they need to do is to replicate a similar process. First you move all the indie games and source engine games to linux to "bootstrap" your market. Maybe make them free or a few bucks. Once you have a paying userbase from that, you show financials to bigger fish in an attempt to draw them in. Lather, rinse, repeat.
Even better is the fact that publishers love "free money" that doesn't require them to gamble. A port of an already successful game is much less risky than a new game. And (conditions permitting, like engine support) it costs a very small amount compared to the actual budget. So they can work on smaller margins to still turn a healthy profit.