Hosts with randomized addresses are likely to have auto-generated PTR records, or none at all, so for the purpose of rDNS resolution those are not a big issue.
And that’s a detail, but SLAAC as in RFC4842 is deterministic. The randomization is introduced by the privacy extensions in RFC4941.
Even if we scan just the first address of each /64, it’s still about 2^56. Unlikely anyone is ever going to do it.
This is another thing I like about IPv6. Makes mass address scanning completely useless.