I've never heard of this guy at all. Does he have any credibility at all?
If so, founder of smoothwall, history in the UK security scene. Not to say that means that what's happening here is as serious as what's being alluded to, but has some background in the area.
* https://www.thesslstore.com/blog/third-party-content-injecti...
These risks however aren't a major concern for me, and people who choose to send me mail don't assume security or deliverability.
I guess this would filter non-technical people, but Let's Encrypt existence has made any argument against web certificates moot. There's no sensible reason not to.