It's BS speculation from the tin-hat wing, frequently cut-and-pasted on this site. At Google there is a finite time between when you delete something and when it has to become unreachable through all frontends and APIs, and when it later has to cease to exist, and then finally when the last backup of the data must have been wiped. Source: I personally had to implement the monitoring and alerting that ensures this is the case for a major, privacy-sensitive Google product.
However you should note that if you used a managed Google account, such as educational or corporate account, your ability to truly delete anything may be curtailed by your domain's administration.
It's not a totally insane belief to have. Facebook infamously held onto a "deleted" picture belonging to an Ars Technica writer for well over three years. [1]
Hanlon's Razor would just dismiss that as "CDN's are hard", but it doesn't explain why the one picture owned by the author that they emailed Facebook about finally went away, but others they had the URLs for did not.
Whether it be malice or stupidity, I think the sane default belief is that "delete" doesn't actually remove the bits in most cases, and that goes double for PII brokers like Facebook and Google. Act accordingly.
However you should note that if you used a managed Google account, such as educational or corporate account, your ability to truly delete anything may be curtailed by your domain's administration.