The part about you assuming he obeys the normal laws of self-recovery was mostly a joke meant to lead into what I wanted to say, sorry if I offended you.
You didn't offend, but there was no indication of a joke of any sort. Remember, it's the Internet and people have said both smarter and dumber things that either of us, and been seen as joking or trolling despite that being the wrong conclusion.
It's not so much that they're the same, but rather that they're both inside MC's body. Inside is inside.
They're different kinds of inside though, with different effects and ways of affecting the body.
In fact, it might be easier to heal radiation poisoning than it is to heal certain toxins, because the poisoning is actually part of his cells, rather than liquids that spread throughout the bloodstream.
A small, (possibly lethal after many years to a normal human) dose, absolutley.
Higher doses, particularly of neutronic radiation (Alpha/Beta/Gamma wouldn't penetrate his armoured form) have harsher, faster acting effects.
Assuming his regeneration/healing requires concsious direction/guidance to deal with this kind of damage, even for only a portion of the total process, he'd need to stretch his awareness of the damage and scale of it according to how much exposure he's had. Aberrant cell mutations and defective replication are likely to interfere to varying degrees or possibly even be exacerbated by the regenerative process.
Given the MC doesn't have the same kinds of knowledge or experience as Xanthe or even Ella when it comes to dealing with new (to him) kinds of dangerous biological interactions and effects, it's likely he'll waste time having his body fight back in the wrong way at first.
That kind of mistake could lead to having to rewrite/repair genetic and cell damage in sections or possibly even just hack the worst effected parts off/out and make repairs like he was regrowing parts instead.
TLDR; Like KingAgamemnon said a few posts before yours.
Does every conversation with you have to turn into an argument? You can disagree with what people are saying without taking personal offense.
Again, not offended.
People too often confuse or forget that emotional/psychological offense is not the same thing as being on the end of borderline aggressive and combative (accidental or otherwise) writing.
Someone can see the word aberrant and be immediately offended and conflate it with equalling abhorrent, despite the word being used in a completely neutral and factual manner, but they won't give a shit because they were offended.
When someone's writing comes across as more of a metaphorical "assault" or (indirect/non-personal) attack, that doesn't automatically make it personally offensive.
I generally go into these things prepared that it will degenerate into a mini shitstorm. It's the Internet after all.
It doesn't usually happen, and when it does it's always down to the same factors and attitudes of both involved.
When the other party forgets that they're not talking but are communicating through text only, it usually turns into one because they assign tone and intent where there isn't, and act like their own writing magically conveys their tone and they're somehow an expert in reading shit that wasn't there in the first place...
It also doesn't help when you forget who you've interacted with, how your own side of ther interaction(s) were handled and could be seen as coming across (especially when you've been explicitly told such by the other party).