Category error.

The problem here is deterministic. *It must be for accuracy to even be measured*.

The model isn't trying to solve the Collatz conjecture, it is learning a pretty basic algorithm and then doing this a number of times. The instructions it needs to learn is

  if x % 2:
      x /= 2
  else:
      x = x*3 + 1

It also needs to learn to put that in a loop and for that to be a variable, but the algorithm is static.

On the other hand, the Collatz conjecture states that for C(x) (the above algorithm) has a fixed point of 1 for all x (where x \in Z+). Meaning that eventually any input will collapse to the loop 1 -> 4 -> 2 -> 1 (or just terminate at 1). You can probably see we know this is true for at least an infinite set of integers...

Edit: I should note that there is a slight modification to this, though model could get away with learning just this. Their variation limits to odd numbers and not all of them. For example 9 can't be represented by (2^k)m - 1 (but 7 and 15 can). But you can see that there's still a simple algorithm and that the crux is determining the number of iterations. Regardless, this is still deterministic. They didn't use any integers >2^71, which we absolutely know the sequences for and we absolutely know all terminate at 1.

To solve the Collatz Conjecture (and probably win a Fields Metal) you must do one of 2 things.

  1) Provide a counter-example 
  2) Show that this happens for all n, which is an infinite set of numbers, so this strictly cannot be done by demonstration.