Well, Google is explicitly listed (about halfway down) as paying a percentage, and they're the only ones that are. In my (very limited) experience, it's generally been a flat rate regardless of salary, so I'd go the other way and believe that the majority do, indeed, do that.

Your second point is definitely a major concern, though - the author talks about it (calling out Amazon and Twilio as particularly bad), but doesn't provide any sort of hard data on what the workload is like, possibly because it varies heavily even between teams or groups within the same company.