They're making a joke about how Matrix e2e is so high friction and unreliable that even if you should have access to a channel you'll get "unable to decrypt message" errors from key or identity issues - not that they would be rightfully unable to be decrypted by third parties (all the e2e programs they mention have that property).
Oh I see, thanks.
Sorry, I wish sarcasm had a font face so I could make it more obvious.
Oh yes, I remember having this issue many times before. :D