Unfortunately, Sailfish OS uses a proprietary (closed source) android compatibility layer, as well as a closed source UI.

For the parts they have open-sourced, they implemented a CLA that contributors must sign. It's the HA-CLA-I-ANY license, which specifically allows them a perpetual Copyright and Patent license, and permission to re-license your code contributions to a more restrictive license which enables them sell or package it into a closed-source proprietary app.

Personally I'm be more comfortable supporting the development of PostmarketOS instead, since it is completely open-source with no CLA, meaning no chance of any rug-pulling in the future.