The point is, that there is no sound scientific basis for claiming "it is all just known Physics" at this time
Since everything, literally everything, we think we understand today has fallen squarely into "100% just known physics", yes, we can have pretty high confidence that the things we learn tomorrow will do the same. I do agree it is (vaguely, hand-wavingly, extremely low-order probability) possible we might need some new physics, but given the physical constraints of our fleshy machinery, (a) it seems really, really unlikely and (b) without discovering a mechanism that requires same, there's little point in claiming that is the case.
At various points in time we didn't understand X, but later on, we did understand X, and every time that threshold is crossed, the answer has been "100% known physics." To say that because we don't understand Y yet means "might not be known physics" seems to slyly imply that it might not be physics at all, which our experience with reality does not support. Just in case you were leaning that way.
While it would be magnificently interesting to find something that does not fall into that classification, no one has done that yet, and there's no particular reason to expect anyone to, either. Because it has never happened.