Definitely not mostly Android. It’s their own Linux distro/flavour with an “Android compatibility layer” like Waydroid and (gradually open sourced) system components. Ubuntu Touch has a similar approach and I believe postmarketOS as well. I really hope that Jolla’s next community device brings them some more traction and subsequent development velocity. I tried installing SailfishOS on my Fairphone 5 but it is not (yet) a polished setup experience from first boot. If they can polish that, I think it’s a great OS to facilitate a gradual move from privacy-hell. It would allow absolute must-have apps to live in locked down Android compatibility mode while there’s no viable alternative and more and more of your data living in a tracking-free OS.
AFAIK they offer a way to port their OS to Android devices by re-using the drivers built for Android when building a SailfishOS image for non-native hardware. I personally wouldn’t call that “it’s mostly Android”, but that might be where we have different interpretations. Your initial comment came across as if it was a glorified launcher or a deGoogled Android. If I interpreted that wrongly, my bad. Cheers