That import will only work for players that have been previously exported from the same league file. It matches things completely based on the internal OOTP player ID.
OOTP does not have functionality for exporting rosters from one save and importing them into another save. That has never been possible.
If you are working specifically with historical players there is a different place (the Free Agent screen) where you can import them from the Historical database into any save. This includes the ability to put them onto a team in your Save (using the Team abbreviation).
I posted screenshots for this area recently in this thread.
https://forums.ootpdevelopments.com/...d.php?t=366825