Just trying to avoid the timeout by avoiding redundant calculations across many tests.

The cache will save over time because you only run the first line of code just once, after that all the test are using by calling the function with different numbers.
Using the dict CACHE helps u jump over test you have already done in the previous so it s faster.
Chances are thinked the same way he did but by not using cache you are wasting time on useless tests.