This only makes sense if you ignore the world outside the Bay area and assume it's a talentless wasteland. Bay area salaries are vastly inflated in terms of value for money.
There is lots of talent elsewhere of course. I live in Europe. Lots of smart people here. I think I personally know quite a few people that could do at least as good a job as Signal has at building a messenger app + platform. No offense, but this isn't exactly rocket science.
And of course the elephant in the room here is that money is running out because this organization has a cost problem. Inflated salaries, insane cost for things that they should arguably get rid off (like the SMS bills), etc. That's a leadership problem. They aren't even getting value for money despite those salaries.