Ummo was a psyop. More specifically it was a psiop. Ummo wasn't real. Then the psyop happened and was effective enough to manifest proper alien encounters using the collective underlying psi field, or whatever it is called, of the region. As of that moment in time, going forward Ummo was physically real and retroactively Ummo's history was too.

This was a test operation for a facet of psi theory, before that there were only ever small scale events that were attempted.