Google cache does expire so it will forget eventually. The problem is when you go to the original on reddit, we're seeing that some of the comments still have their text intact, but with a [deleted] username now.
This is the expected behavior on Reddit when you delete your account. None of your posts go anywhere. You have to manually, before you delete your account, edit them to remove their contents. Requests for deletion under GDPR may function differently.
Several folks here have reported this noncompliance to their gov't and i expect an epic showdown where reddit gets its behind handed to it, but that's likely some years away.
The important thing is to get people to be aware of this. A lot of folks seem to delete their accounts and then, too late, be caught by surprise that nothing they wrote is deleted - and now cannot be removed.
Some folks do the right thing, but then get surprised when it turns out that redact.dev or shreddt.com or Power Delete Suite failed to delete all of their comments for various reasons.