Exactly, you do have to gain some understanding of Postgres yes, but it's SQL at the core which IMO is what you want 90%+ of the time, and you're not locked into their platform. When your company gets larger and you're ready to start wasting VC money on db admin and other problems that have already been solved, you can rip out Supabase and all the SQL will still work.
I did the nand2tetris on Coursea, it's what started my passion for software, I was just tinkering and hacking before that, did that course and knew without a doubt it's what I want I want to dedicate myself to.