tmpfs is the name of the RAM based filesystem, but it doesn't have to be mounted on
/tmp or even used for traditional "temporary files". The example you've pasted is mounted on
/dev/shm, which has nothing to do with
/tmp, and so
/tmp is probably just real disk.
Linux has no built in mechanism for
/tmp "oveflow", so you'd have to set something up manually to achieve this. There is some distinction between
/var/tmp tends to be used for bigger files), so mounting a
/tmp and leaving
/var/tmp on real disk may be good enough.
If you actually want it to overflow you'll have to setup a union filesystem like unionfs or aufs. These filesystems take multiple underlying directories and expose them as a single mountpoint. You can configure the priority of the underlying directories so that the
tmpfs is used first, then the fastest disk, etc.