MC says he has no magic power. Proceeds to use a super power that goes beyond the realm of what is physically possible for a human or even a super human and is basically magic.
MC says he trains for immunity to various toxins. No. This doesn't exist. It only happens in wuxia novels. You don't gain "immunity to toxins" no matter what you do. You only gain resistance to toxins. You can never be immune to toxins, only resistant to a certain amount.
The MC was exposed to enough toxins to die, even if he has an "immunity", because that's how things work. It takes time for your cells to get rid of toxin, mean while, being splash with enough toxin to destroy all of this innards before they can even neutralize it.
If he did, then his cells are no long the cells of a human being, but a superhuman if his cells can instantly neutralize any toxin, poison, venom, etc.
Basically, the MC is using magic, despite the author saying his power comes from physics. No. Its a super magic power.