I work for an email service provider. While we usually get a response from the inbox provider that an inbox does not exist, we totally get async bounces all the time. Some providers accept the mail and realize later that they cannot deliver it.

Can confirm. Async bounces make up somewhere around a few percent of overall bounces for most senders.

While true, email accounts can and are deleted or closed. Transient addresses on your own domain are usually the best (so you can nuke them when polluted, hat tip to Apple for pushing blind emails into the mainstream with "Sign in with Apple"), but sending fake bounce backs by sender while binning anything incoming from them is a close second (Gmail and Fastmail both support filtering messages directly to Trash, but no fake bounce back messages; could probably do it with an SMTP proxy, again if you use your own domain).

I think they're talking about sending an error at the SMTP level. Not sure if that will break stuff though.

What GP describes would work the next time they try to spam you, not for the current email you received.

correct. it's for future deliveries