What did I get wrong?  There are other coinjoin services whose transactions don't use OP_RETURN at all and get mined just fine.  You cannot say that the filter is targeted specifically at coinjoins, because it clearly isn't. 

 Op_return is not part of coinjoin txn!
nostr:note1zx8usyfff3xwmc67mwasuycf6dtzggrslpjwpeps7alvrf7glmjslq07pn