Where am I assuming that we have perfect knowledge of physics?

To make it plain, I'll break the argument in two parts:

(a) if AGI is impossible but humans are intelligent, then it must be the case that human behavior can't be explained algorithmically (that last part is Penrose's position).

(b) the statement that human behavior can't be explained algorithmically is about physics, not mathematics.

I hope it's clear that neither (a) or (b) require perfect knowledge of physics, but just in case:

(a) is true by reductio ad absurdum: if human behavior can be explained algorithmically, then an algorithm must be able to simulate it, and so AGI is possible.

(b) is true because humans exist in nature, and physics (not mathematics) is the science that deals with nature.

So where is the assumption that we have perfect knowledge of physics?