Jukka is incorrect here. The W3C validator does not check against the WHATWG LS, instead it checks against the W3C HTML specification. The W3C HTML spec is the authoritative source for conformance checking requirements for the W3C Validator.
In regards to menuitem
not being allowed as per the HTML spec, I believe this is a bug. And as such I have filed a bug. It is now in my bug queue to be resolved.
I have filed a bug against the W3C validator and wai-aria in HTML doc as well. Until such times the validator is fixed, I suggest you use the roles as per the WAI-ARIA spec and ignore the validator.
addendum:
I looked back at history of ARIA integration into HTML could not find any reason why menuitem
was not allowed, so believe it was an oversight. I fixed and resolved the bug I referenced above.