You are absolutely correct, it’s just a good ole lazy migration
In theory, (and don’t try this at home!) if you turn off lazy migration and disconnect your legacy DB from Auth0 entirely, your already migrated users won’t even know since they are authenticated against Auth0 DB now. Unless you also run some custom data fetching from your legacy DB and not 100% of user metadata is already stored in Auth0, there would be no difference from user perspective. This is because your users are not authenticated against your legacy DB; their password and profiles are stored in Auth0, they are authenticated by Auth0, and (possibly) some of their external and additional information is also fetched from your custom DB (but only after they are authenticated by Auth0). It’s a bit of a simplification, but you get the point.
Custom DB feature allows enterprise customers to run authentication against their custom DBs (bypassing Auth0, so to speak, but taking full advantage of every other available feature). I don’t think it’s your case, unless you run a massive enterprise with gazilions of users and a very particular set of