This was very unfortunate mistake on the sender part - he mistakenly wanted to send 2.5 BTC and not include it as a fee.
This transaction fee had nothing to do with current high (but not that ludicrous) fee policy.
I always say: there should be hard coded limit build in every bitcoin wallet preventing you from including ridiculous fee like that.
Also I wonder if AntPool pitied him and gave his money back, even though they have no obligation to do that...
True. As you say, a hard coded limit or even a notifier/warning that pops up should be included in wallets. Something like if a set max/min fee is reached, there'd at least be a warning. Then, if they still send a such a ludicrous amount of fee despite the pop-up warning, that'd just be considered a 'donation' of sorts.
Yes, they did give back 2.5
BTC. The fees were already distributed to the miners, but even so, Bitmain still decided to return the same amount, from their own pocket.