The fediverse is the real internet, it's not a company providing a service. On the real internet, once something gets out there, there can never be a guarantee that it's taken back. Even on Reddit, once you post something, Reddit might fully delete it but someone out there may have copied it.
I had years worth of posts and comments that I deleted via the interface a while ago. Then as part of the reddit exodus I decided to run a removal tool that used the API, and it turns out 11 years worth of "deleted posts" were all still sitting out there, they were just hidden from me.
I did find it strange when I received a reply to a years old comment that my profile page said was deleted, but I just thought it was a caching issue. Turns out all of that content was still out there with my name attached, I was the only one who couldn't see it.
What about editing the comments? Do they keep any log of the original message and the subsequent edits or something? Maybe this would be a workaround to effectively delete them.
There's no way to know without proper testing, and I'm gone for good.
I did use redact.dev, which overwrote all of my comments before deleting them, so fingers crossed that the account is nuked.