I imagine it only officially supports ASCII because it will be migrated away from MzScheme eventually.
Note: Those "u"s are supposed to have umlauts, but that's apparently normalized away somewhere. The point is, u with an umlaut is treated as a single character by the current implementation.
I'm sure it'll be agnostic, if by "agnostic" you mean that it just reads in strings as a sequence of bytes. It would be easier to do that than to check for non-ASCII characters and handle them specially.