It's clear how this selector "p.foo:nth-child(even)" work, like you describe, and why you put this ":nth-match(even p.foo)".

But this two selectors:

:nth-match(even of p.foo)
p.foo:nth-of-type(even)

don't have the same result?

So only improve for this new pseudo is that whe can use selector list:

:nth-match(even of p,div) {}
the same as:
:nth-match(even of p), :nth-match(even of div) {}
p:nth-of-type(even), div.foo:nth-of-type(even) {}

or :nth-match(even of p,div) means, take p and div (it would be one sub sibling list) and start indexing. This will be nice, but when I see that selector list is treated as union, benefit is less. Or add new variant (conjunction) for argument in functional pseudo, like p^div, p&div, or sth similar.