I don't think they are. The use of "RNG" is referring to how a "random" commit sha lead to this bug. Commit SHAs are, for all intents and purposes, random.
You could test this against thousands of SHAs and never encounter one that meets the criteria to trigger the bug.
To get this specific bug - one 'e' in the 7th digit and 9 digits less than 'a' - is 1/16*(10/16)^9 = 0.009095. Just under 1 in 1000. But if the 'e' is in the 2nd,3rd,4th,5th,6th you'd get the same bug, so actually about 6 in 1000.
But around 1% of the hashes will be all decimal digits and will parse as numbers ((10/16)^10 = 0.009). That's common enough that I've seen that one cause errors in our code too (somebody had code reading the hash then trying to append it to a string).