As far as I know this is not accurate. In April 2021 I made a $16,000 tax payment through PayUSATax.com, and it immediately triggered the new account bonus on my American Express card.
I did a tax payment in April as well and sadly the system marked as a cash equiv and saying I didn't meet the requirement. Just talked to their agent and opened a case recently... still waiting for their response. But that's what the agent saw in the system. So YMMV maybe.
I did the same for several times before and it all worked.. not sure what happened this time and they said there was no rule changes recently.