Comment 1 for bug 1866634

I should state this differently. snaps *do* have access to a '/tmp' directory. but it is intentionally a *different* directory than the host machines' /tmp

See places like:
https://forum.snapcraft.io/t/temporarydirectory-and-how-to-manage-access-to-tmp/10176
https://forum.snapcraft.io/t/sharing-files-via-tmp/1613/20

Regardless the fix is still that you cannot use '/tmp' for files that you want to share between snaps and things-that-aren't-that-snap.