I'd think some humbleness would be appropriate from the, let's call them physicalists.
I'm inclined to consider myself a physicalist.
That being said, I think that humbleness is very appropriate in at least two different ways.
First, I'm not even close to having satisfactory answers to many questions about this physical universe. (And I'm rather skeptical about the more speculative assertions by theoretical physicists about what the answers might be.) To me, inquiry is kind of like peering into a thick fog with a weak flashlight. We can see a little ways, but we can't speak with total assurance about the unknown things might still exist out there. (The unknown is an unbounded set, simply by definition.) I'm a physicalist because the only reality I know is this physical universe, but having said that, I think that this physical universe remains profoundly mysterious.
And second, even though this physical universe is the only universe that I know (and seemingly the only one that I can know), I can't rule out with absolute certainty the possibility that other kinds of being or reality exist in addition to it.
I say, not so fast! Who says rationality is sufficient to understand the universe? I have deep reservations.
So do I.
A cockroach lives its life very effectively, despite being totally unable to understand, or even suspect the existence of, Schrodinger's equation. Cockroaches simply lack the cognitive equipment to understand everything that we think is important in comprehending reality.
So who's to say that we humans occupy the pinnacle of all possible cognition? Perhaps there are superior space-aliens out there who stand in the same relation to us that we stand to the cockroach, aliens that can imagine and comprehend aspects of reality that will forever be beyond the grasp of mere humans like ourselves.