First of all, thw clip does **not** state that this problem was used, especially not used as a test for reliability.

Next a sort look at its Wiki entry would have revealed that for one the first problem used - and indeed used to prove reliability - was to find the highest (all) dividers for a rather large, but still manageable number, in this case 2^18:

> The first of three programs written for the machine found the highest proper divisor of 2^18 (262,144), a calculation that was known would take a long time to run—and so prove the computer's reliability—by testing every integer from 2^18 downwards, as division was implemented by repeated subtraction of the divisor. The program consisted of 17 instructions and ran for 52 minutes before reaching the correct answer of 131,072, after the Baby had performed 3.5 million operations (for an effective CPU speed of 1.1 kIPS).