I can confirm that this is the case. I provided a gibberish answer to a security question for Blizzard. I didn't bother to write it down, relying on not forgetting my password.
I never forgot my password, but Blizzard shut down my account anyway because I was making payments with a card that was not listed as the account's "primary payment method". (The card I was using was listed on the account, but another card was the "primary payment method".) When I had to call support and answer my security question, the answer I'd filled in just meant that I wasn't required to provide the correct answer.
This way "it's a bunch of gibberish" doesn't get past their security.
Never had to use these for real yet, but it should be a bit harder to be seen as a “a bunch of gibberish”.
Remember, an attacker can call support hundreds of times, getting a different rep every time. There's a good chance it'll work eventually.
Random but outwardly appearing valid ones are fine (but you'd want to avoid using the same answer on different sites). One site's "first car" could be Porsche 911, another's Aston Martin. Both aren't true, but the support rep doesn't know that.
Lately, I've been making up a seemingly correct, but random response (and different each time). My favorite vegetable? Sea cucumber! I store that in my password manager.