Nowhere near Primarch level.
Primarchs are so powerful that as babies they can be hurled through the warp, have their capsule explode on an alien wasteland of lava/ice/plague and as an UNARMED BABY defeat a whole squad of enemy fighters.
In comparison, Phoenix Lords are complete pussies.
Primarchs can go kill dragons in hand to hand combat naked and there's no evidence Phoenix Lords are any better than Primarchs at tactics or strategy.
Even more particularly telling when you compare in-game stats.
Phoenix Lords are comparable to non-Primarch Space Marines.
Marneus Calgar, Tigurius, Lysander, Sicarius, Shrike, Vulkan, The Sanguinor, Mephiston, Dante, Astorath, Azrael, Ezekiel, Logan Grimnir, Njal Stormcaller, Ragnar Blackmane, Drago.
Many of these have a combination of superior stats, superior weapons, superior armor, superior special rules to a Phoenix Lord and they're simply weak Children of the mighty Primarchs.
It is not to say Phoenix Lords are puny to say they're power is relative to a normal generic Space Marine Lord. Space Marines are already super-humans. Phoenix Lords are definitely nothing compared to Primarchs though.