Distributing your bitcoins in more than one wallets will save you in the future and El Salvador knows this.
No, this is not correct. Don't make declarative statements on topics which you do not understand. Distributing to more addresses or wallets does not do anything to safeguard against quantum computers. What is important is whether an address has been reused or not.
My understanding was that the important thing is not to have spent from that address, not to reuse it (such as receiving bitcoins multiple times at that address without spending them). Is that so?
I also see a slightly lower risk in having your holdings in various addresses because the attacker would have to search for them, rather than finding just one. Although the essential thing here is protection against attack.