I really don't understand what is his usage pattern would have triggered that obviously automated ban. Can somebody let me know what they might think is adversarial enough to be considered 'hacking' or similar by a bot?

Google is dealing with a wave of abuse over its Antigravity IDE, with 'account switching' tools designed to use a ton (20+) of free or pro accounts, giving the user essentially unlimited usage. I'm guessing they've deployed some rather aggressive countermeasures to stop this, including banning clients that seem to be accessing "private" APIs outside of a Google product.