My guess is that the SPAC does the S1 filing before making the acquisition. Therefore they don't need another S1.

Out of curiosity, how onerous is the S1 filing? Is it any more difficult to fulfill than the annual reporting requirements?

No idea on how complicated it is to file an S1. I just type code :)

From my understanding, the SPAC goes public (with an S1) with the intent to acquire "some company". Then the company being acquired doesn't have to file an S1. I would guess that the SPAC's S1 is going to be a lot less complicated, as it's a do-nothing holding company set up to acquire some TBD company.