you are right. The feet are made of the same material as the base (OSB).
When feet touch the floor, water drains out by the slope designed in the tub; that is best kept as designed. So mortar is best when around the feet and not under them. Enough to solidify that central piece of OSB, that is all.
Without mortar the tub will still hold up too; mortar is just more support. Walk around in the tub in socks or bare feet and you may hear minute creaking or cracking sounds -- that is the beginning of little stress cracks that will be visible years in the future.