"Omega level mutant: a mutant whose dominant power is deemed to register - or reach - an undefinable upper limit"
Meaning when she accesses 100% of her power she is unparalleled. That don't mean she's always at 100% or even most of the time. Xavier base abilities may even be stronger then hers (he battled Dark Phoenix to a standstill if I remember correctly and even almost overpowered her) but when she reaches her full potential she's above him. It could even only be about Phoenix Jean for all we know. That data page don't have enough information.