It should support it just fine. Sometimes there is an issue with not being able to figure out the syntax type and you need to pass a flag to tell it the correct type to use but that doesn’t appear to be the case here. I’m guessing there is something wrong with the file. rdf:about is fine to use but it’s probably in the wrong place or there is an earlier error and it’s just failing there.
Can you post the file? I’ll see if I can spot the problem.
I'm not sure if it's a copy/paste error, but your ns2 seems to end with an &, which makes it an invalid namespace. It's also missing the end quote, but I'm 99% sure that discourse just ate it