Not a bug.
Any ability that is pulled from another class is available regardless of what level your current class is at.
So yes, a Gladiator at level 1 does have access to Stoneskin, while CNJ/WHM can't use it until level 34, and won't be able to use it when level synced.

The same applies to crafting classes.

I don't remember what the dev answer was when it was brought up last time, but it's considered as working as intended.