That is what dnf says, but what it actually does is install of new and removal of old.
Since the new is a 8.9 version from 8.9 repo that does not have any 8.8 version packages, you cannot downgrade.
AFAIK, all support for 8.8 did end the moment the 8.9 was released, so effectively there is no 8.8 any more.
I said that there might be an archive of ghosts past, a vault: Index of /
One can define additional repositories. If they point to vault that has 8.8 content, then dnf would see the packages again.