Yes, one concern is that humans confuse the old and the new ID.

I also think it makes sense to allow the new interfaces to look up the
mount based on either the old or the new ID.   But I could be wrong

Hm, mount id recycling may happen so quickly that for service restarts
with a lot of mounts this becomes mostly useless...

there, since that might encourage bad code.  Maybe the new interface
should only use take the new ID, which means no mixed use of
/proc/$$/mountinfo and statmnt/listmnt.

... so I think that is indeed the better way of doing things. There's no
need to encourage userspace to mix both identifiers.