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?
That is right. Do not make any sending transactions from the address in which you want to securely store Bitcoin for the long-term. This will protect you from the first wave of attacks if they catch us by surprise. Even in this unlikely case where we are caught off-guard, I can easily see miners offering a submit your transaction service where you pay a slightly more fee in order for them to mine it without it being broadcast to the network. This would prevent it from real-time quantum hacking. However, it is always important to remember that this is all theoretical. Even if there were QC's that could break ECDSA keys tomorrow, we don't know how long it would take much less if there would be any chance to compromise keys in real time (after you submit a transaction and before it is confirmed)
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.
Let's be honest here. Nobody in this thread is rich enough to be a target.
Therefore, we should all saves ourselves the trouble and just focus on a simple setup that maximizes security. One cold storage with maximum security, and one or two spending wallets for frequent transactions.