As long as you set up SPF and DKIM records on your mailserver, you’ll never get marked as spam.

Sorry - that's factually incorrect. If your IP is on a residential block, you'll be downscored. If you're on a dynamic IP, same again, but weighted even more harshly, by pretty much every antispam service. In addition, every commercial service is very secretive about what methods they use, for good reason, so you cannot claim with any accuracy that "you just need to do this $thing to get read". (Although I do agree the original post is not well researched, knowledgeable nor particularly useful to anyone)

SPF and DKIM are essential to getting your email out, but it's not the only thing, and sometimes no matter what you do do, your hit rate is going to be low.

Source: Me. Been running mail servers privately and commercially for over twenty years. Before then, I ran fidonet and netmail services through the 90s and into the tail end of the 80s. There's many things I know bugger all about, but email is not one of them. (And if anyone's interested what I do for personal email now - I use gmail, because it works and maintaining it is somebody else's problem)