1) I don't think they mean "scale your projects elegantly" in a "distribute your project to millions of customers" sense, but rather in a "add lots of libraries and not have it be a hassle" sense.

2) It makes perfect sense to let GitHub handle the performance hit until issues arise. Premature optimization is the devil, right? But once there start to be issues, it's definitely unfair to turn around and say "well you offer the service for free, so you should fix it"