«أُولَئِكَ عَلَى هُدًى مِنْ رَبِّهِمْ»
Those are established upon guidance from their Lord.
The ordinary spatial relation of «عَلَى» is being upon or above. Steadfast possession of guidance is conceptualized as securely being on top of a mount or support.
The people are not physically seated on guidance. The relation of elevation or support is transferred to confident adherence to guidance, illustrating derivative metaphor in a particle.