Article says you cannot side load books on Apple Books. That is incorrect. You just send an epub to books via the share menu on Mac or iOS and it loads it. Also syncs it via iCloud if you want it to.
Perhaps the author meant you cannot download purchased books off of Apple Books.
I get not wanting to use a google, microsoft or crypto laden browser, but I would be willing to use a well supported browser that used chromium as the page rendering engine. It seems to be extremely difficult to get another engine to be competitive in the marketplace. Maybe the resources would be better spent putting the chromium engine inside a different container. I’m sure there would be drawbacks, but I think there would be compatibility benefits too.