By your own story, it's not a myth. Not everyone has one, and your boss had to be supportive of you working on it, but there are a bunch of internal systems that are supported by 20% time. It might be more appropriate to frame that 20% as giving 120% (aka working on it on Saturday instead of Friday) for some of the things, but it's a real thing that exists. Just because not everybody has such a project doesn't make it a myth.