> Aren't donations untaxed?

I don't know every jurisdiction, but that's not the only relevant factor. If they are tax-deductible, they still introduce the need for paperwork etc. for the payment processor to let the payer deduct it from their own income. A 0% tax on some action doesn't imply you don't have to worry about tax law when performing said action.

Though I think you can just read the policy and take it at face value and not even have to know any of this beforehand.

That’s not true. If a donation is tax deductible that’s just between the IRS, the recipient and the donor. The payment processor, banks, etc don’t do anything special.

But the complaint isn't that Google won't process the payment. It's that they won't allow them to link to an OpenCollective donation page, and presumably OpenCollective would handle anything like that.