SPACs issue units to their founders that will convert into approximately 20% ownership upon the successful completion of a deal
I do think you do raise a fair question of what stake Chamath et. al should get for sourcing a deal. 20% is definitely high, and I think a typical SPAC is something like 5% sourcing stake for the owners.
20% is standard SPAC comp.
Also yes, I apologize, you are right - they get 20% of the value of the SPAC, so if they buy 10% of a company, they are only getting 20% promote on that 10%.