To me, that reads as "if you had 1 BTC stolen on May 20, we will deposit 40k USD into your account, because that was the value of 1 BTC as of May 20", not "if you had 1 BTC stolen, there is now 1 BTC back in your account".
The timeframe listed in the letter covers exactly the time of a massive price spike, so a USD payout would put most people in a better situation than a BTC payout in this specific case, but I'm still curious how this is handled, and whether there is a universally agreed standard for it.
Because next time "we'll reimburse you the USD value of your crypto as of the date of the attack 6 months ago" could mean that someone "made whole" like this has only 10% of what they would have if the attack didn't happen.