You are immune to poison and low-level radiation. Additionally, you treat each other category of radiation as one step lower (for example, you treat high radiation as medium radiation).
Radiation is a poison effect. So by RAW being immune to poison means you aren't affected by radiation, right? Unless the RAI is that radiation doesn't count as a poison effect for the purposes of this discipline?
There are several pieces of Starfinder that indicate that while radiation uses the poison rules, it is not itself a poison for all purposes, including resistance/immunities. See also the Polymorph 6 spell, which includes an option to be immune to either radiation or poison.
I feel like thats likely what it should be. It's pretty confusing when looking at the rules for radiation and it says it's a poison effect.