The government of El Salvador can do whatever they want with their BTC, but a few things come to my mind.
1.There's no "quantum threat" coming anytime soon. Why are they preparing for this so early? Maybe the "quantum threat" is just an excuse and there's another reason for the government to move the BTC to multiple wallets. What if the IMF wants to take control over El Salvador's Bitcoins?
There many not be any quantum threat the moment, but that doesn't mean there wont be one in the future. El Salvador is protection their Bitcoin not for anything but against any future threat. Just because we haven't heard of any attack or upcoming attack that doesn't mean we dont know what the quantum computers are capable of.
There are lots of predictions of how quantum can threating the online space in the nearest future. Do you really think El Salvador should wait until they striker before protecting their national treasure. Since their national treasury is in Bitcoin, they are not meant to gamble with it Early precautions is better than regretting.