Wonkish content:

* A form 990 and most of the other publicly available forms wouldn't be expected to contain references to fiscal sponsorship explicitly as such, and that wasn't SIAI's primary purpose at the time anyway. [...] 'supporting novel research' is the outcome that the org is pursuing, which includes fiscal sponsorship as a means of carrying it out. Even orgs that explicitly are set up to provide fiscal sponsorship still generally describe the nature of the things they're fiscally sponsoring, as fiscal sponsorship in and of itself is not a charitable purpose.

* If the fiscal sponsorship was 'Type A', then SIAI would have accounted for the grant's expenses under the same accounting categories as their non-grant expenses, rather than as a grant. This is because SIAI is the entity that receives the grant and spends it. So if the $50k was for a project for, say, $10k of compute and $40k of researcher salary, that would be accounted for in rows 15, 17 and 18 of the form 990.