Is email aliasing secure (SimpleLogin, addy.io, et al)?

Here’s this Reddit post I found with a quick search. Oddly enough Reddit seems to be down on my end (probably some NextDNS config acting up again) and Archive.org has blocked archiving Reddit pages.

https://www.reddit.com/r/Simplelogin/comments/18qxati/posting_public_key_for_alias_with_pgp_enabled/