I believe the development team sometimes add features to Aeon first before adding them to Monero making sure that they are stable and useful.
As far as I know that has never happened and AEON is
NOT a testbed for Monero. In fact in the near future it will be the exact opposite. Aeon will integrate stuff developed and tested in Monero (lmdb and GUI at least, don't remember if ringCT).
This project was picked up by smooth alone (after the former dev closed shop) and although there is some overlap in the community, the Monero Team has no official ties to AEON.
I agree that there is no official relationship despite the overlapping community and smooth as a developer.
However, Aeon improvement have already been merged in to the Monero codebase.
Here is an example of a privacy improvement found and fixed by smooth for Aeon before it was merged to Monero