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.