Wasabi could compile VBScript, which was the language they had at the time. It's actually a more or less identical approach to the one Facebook took with Hack, but Facebook had a lot more resource to make it awesome.
Yes, definitely there can be valid reasons to write your own language/compiler etc. Of course, the size of the Facebook code base, not to mention engineering team, put it in a clear class of its own. And the Hack project was run by people with extensive experience in compilers and type systems, not a summer intern... hah. But yeah, it’s not a bad idea in all cases. Facebook, google, Microsoft and many others have invented new languages and/or written compilers. And I’m sure there are valid reasons for a small organization to do it as well — but I fail to see what made it justified in this case. Then again the product didn’t go belly-up, so there’s that.