They believe that through this, they are reducing the risks of losing all their bitcoin should quantum threats be real in the future and their holdings get attacked. That is the only reason why El Salvador is moving their bitcoins and I believe this should be something we can all learn from El Salvador. Distributing your bitcoins in more than one wallets will save you in the future and El Salvador knows this.
Even though i don't think quantum computing is a real concern nowadays, but i still think it's wiser to split the bitcoin holding into multiple addresses for various good reasons. One of them is protecting privacy and reduce tracability.
Quantum computing is so early to talk about or to take security measures against. One day in the future, when this technology will become accessible to manipulate the blockchain, bitcoin or all other blockchain based cryptocurrencies will lose value fast than anyone may expect.