Biologically, nitrates and phosphates are used up in accordance with the Redfield ratio. That is, carbon: nitrogen: phosphorous, 106:16:1 ratio. So having zero phosphates would indeed make it harder for bacteria to reduce nitrate. However, it is usually the case that zero nitrates detected on test kits merely indicate that the levels are low, rather than phosphates actually being completely absent. The biological process should still be occur.
That being said, since this form of reduction of nitrates and phosphates occurs biologically, it should not affect the ability of purigen in removing nitrates.