If you read https://www.freebsd.org/cgi/man.cgi?query=tmpfs , you will notice that this page doesn't mention that tmpfs can be mounted using either the ro or rw mount option.
(In reply to Oleg from comment #0)
Shouldn't that be a mount(8) documentation piece? It is not specifically about tmpfs(5), is it?
mount(8) and fstab(5) both document "ro" and "rw".