Why do they say it's white hat hackers?
Purported white-hat hackers. The implication is that these people are presumably describing themselves as such, but it may not actually be true.
The same reason scam artists describe themselves as businessmen
Presumably to try and reduce the chance of a mass panic among their users.
While moving the funds, the attackers left an OP_RETURN message with the text "we are whitehats. contact us on chain" https://mempool.space/tx/c103de95817b43f2df635ec6f35ff126ca2...