True overall, but I think it makes a lot of sense to evict rarely used data in tmpfs to swap, so that the DRAM it occupied can be used for valuable caches instead of holding some obscure temporary data that will be rarely or ever accessed.
Rarely used data that got evicted then behaves more or less like a normal /tmp filesystem when it does eventually get accessed, i.e. it gets read in from disk, while other data still gets all the benefits from tmpfs (e.g. ephemerality).
(If you take the thought experiment to its logical conclusion, you'll anyway end up in transparent hierarchical storage a la AS/400, where all data is just addressed by a single pointer in a very very large address space and the OS decides where that currently points to, but let's stay within the confines of what we're mostly used to...)
Rarely used data that got evicted then behaves more or less like a normal /tmp filesystem when it does eventually get accessed, i.e. it gets read in from disk, while other data still gets all the benefits from tmpfs (e.g. ephemerality).
(If you take the thought experiment to its logical conclusion, you'll anyway end up in transparent hierarchical storage a la AS/400, where all data is just addressed by a single pointer in a very very large address space and the OS decides where that currently points to, but let's stay within the confines of what we're mostly used to...)