Seems to me their server settings are wrong. I can delete every email on my iPhone, but it does not delete them from my server. So when traveling, I keep my mailbox rather clear by deleting 99% of my emails, but once home, they all download from the various accounts, get sorted out by the Rules I've set up over the years.
Even if they are deleted when wandering, they still reside in the Trash, so are not really gone, just moved to the Trash folder...
That author is not understanding how it works.. Still works..
Just updated ourselves to 11 Pro & Pro Max!
Fun stuff to sort to, better for my wife as her fingerprints even flunk the TSA tester! So facial recognition solved that!
weco