[timwiseman]

I think he means that even in a purely classical system (ignoring relativity and quantum theory), F=MA describes a system you could never see in the physical world. After all, it does not take into account friction or constantly acting forces like gravity (yes, I know gravity's nature is more complex than that, but I am talking about a purely classical system).

And that is if we can ignore relativity and quantum theory. With those, it gets more complex, especially given that they cannot currently be reconciled with each other.

As others have discussed, F=MA and all of Classical theory is a model that is definitely wrong, but it remains close enough to be extremely useful even now.