That US$100k seems to have arrived over the last year, and their overheads - from quickly looking over their OpenCollective expenses tab - seem to be mostly buying hardware for CI things, and a monthly GSuite bill.
It's also pretty likely they get a lot of their resources for free / sponsored too.
https://opencollective.com/homebrew#category-BUDGET
They seem to be receiving about US$2k/month via Patreon too:
https://www.patreon.com/homebrew
I think their Patreon was around the same when I looked ~12 months ago.