In this scenario, Amazon doesn't want to contribute to Elastic, Elastic wants Amazon's changes from Amazon's fork (Opensearch). So they can't demand Amazon sign a CLA, because they can't offer Amazon anything for it. Amazon is fine just ... not signing and continuing on their open fork.
Elastic have lost the Schelling monopoly on what constitutes the "mainline".
But Elastic can already take the changes from Amazons fork - opensearch is permissively licensed. What they might want is that Amazon stops maintaining its fork and contributes directly to elastics mainline. I believe that ship has sailed, and it's not coming back to pick up stragglers.
Elastic have lost the Schelling monopoly on what constitutes the "mainline".