They have an explainer on "how" or "why" it is free. Basically Serif was bought by Canva. Canva, which is subscription based, has a turnover of $3B USD, and they are basically betting on that if they make it easy to use Canva, via compatible software, that software might as well be free.
This means you can (optionally) sign up for Canva Pro (if you'd like to use AI functionality in Affinity)...but it also means if you don't care much about AI stuff you can just keep using the software without it. For free.
A bold bet, but hey...it is free for you and me...
