https://pubs.opengroup.org/onlinepubs/9799919799/
It doesn't get more standard than that.
It's because of people doing random nonstandard shit that we need to Docker-ize a lot of software these days. People refuse to lift a single finger to adhere to conventions that let programs co-exist without simulating a whole god damn computational universe for each damn program.
Back in the day you might place /tmp in a good spot for random access of small files on a disk platter. /var is vaguely similar but intended for things that need to be persistent.
Anyway it's not uncommon for systems to persist /tmp and clean it periodically from cron using various retention heuristics.
Ultimately POSIX concepts of mountpoints are strongly tied to optimizing spinning rust performance and maintenance and not necessarily relevant for SSD/NVME.
It doesn't say anything about what it's backed by.
I'm not really seeing a right or wrong here anyway unless you're distributing a script that's meant to run on all sorts of Linux systems. In which case you probably aren't concerned with the physical storage medium being used.