You've misread those other threads. Multiple collections can be synced.
Not sure how to quote another thread–but this is what I read:
"Did you connect a new Lr Desktop catalog to your account? Since Lr Mobile currently only supports one catalog at a time the previous connected catalog will be purged. In that case you should have recieved a caution allert within Lr Desktop. - Guido"
If I am misreading something then I would very much appreciate assistance.
Yes multiple collections can be synced between Lightroom desktop and Lightroom Mobile.
One catalogue, but multiple collections. Try to sync another catalogue and you'll remove any other collections from syncing.
Well, I guess I'll have to take your word for it even though there is no trace of my first collection on LR mobile.