in this cartoon, the lady character says : “Easy - - just memorize it as the first three prime numbers, if you realise you couldn’t remember what came after 2 and started to panic and get them increasingly wrong”
first 3 primes : “2, 3, 5”
forgot 2nd : “2, …, 5”
then, get the next primes : “7, 11”
… increasingly wrong : “not 518 ? ?”
No, either it’s a bad one or I’m missing something.

https://explainxkcd.com/wiki/index.php/2937:_Room_Code
Cueball and Megan are on vacation or otherwise traveling, and receive a code for their hotel room. Megan gives a seemingly nonsensical and unhelpful mnemonic by which Cueball can remember it… which, inexplicably, actually helps Cueball to remember the code. Cueball becomes VERY angry on realizing this.

The first three prime numbers are actually 2, 3, and 5. So this technique easily identifies the first two digits “02”. “05” is slightly wrong because it’s not the second prime number, it’s the third. And “18” is more wrong because it’s not actually a prime number, it’s 2 x 3 x 3.

So the mnemonic itself doesn’t really provide the method for remembering the code. Instead, figuring out how to apply the bogus mnemonic will reinforce your memory of the code.