I was able to pull this off, but it was harry. Basically your LDAP users are not regular users at all and probably don’t even have user ids, they have UUIDs mapped in LDAP to a user id.
My solution was a very involved set of SQL hacks to find and replace all the UUIDs with usernames that matched by SAML provider so I could use it directly without LDAP’s help. This is not something that Nextcloud supports out of the box.