/tmp isn't stored in memory; it's usually a normal on-disk filesystem that's cleared regularly. You want /dev/shm instead, which is a purely in-memory filesystem on normal Linux systems.
The point they were trying to make is that it doesn't have to be, and it isn't in several of the Linux systems I've used over the years. Assuming that it is is a bad idea.
It is if your system uses tmpfs for /tmp
https://en.wikipedia.org/wiki/Tmpfs