I had to deal with them a few ears ago. When a client’s bad DNS change was made and then fixed, iCloud didn’t pick up the fix and so wasn’t receiving email for this domain. We contacted them and had a brief conversation with an engineer (similar process as in the article, except someone reached out to us to let us know they made the change.)
If we hadn’t suspected failure to adopt that DNS change, I suppose a lot of people would have thought iCloud was doing silent filtering.
I can’t say how many reasons contribute to the silent filtering problem, but based on our experience iCloud has at least one architectural deficiency that creates a problem iCloud is too big to still be having.
I had it happen once dealing with a company. They sent a confirmation email, I never got it. So I had the system resend it a few times. Nothing. I contacted them, and they said their emails never go through to iCloud addresses, so use a different one.
That’s the only time I know it happened, who knows how many times it has blocked stuff I wanted? That said, important stuff I was expecting has always gone through. I keep saying I am going to switch to Outlook full time, but I have been too lazy to actually do so.