I can't think of anything _exactly_ like that, but I think you can get close by just copying some type of image file to /tmp and then moving it to disk when you're done after unmounting.
/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.