It's explained in the article, but the TL;DR is that CF and DNSimple are simple pretending that a CNAME on the root domain is the corresponding A or AAAA record

It's explained in the article, but the TL;DR is that CF and DNSimple are simple pretending that a CNAME on the root domain is the corresponding A or AAAA record instead. It breaks geographical CDN a bit but it works somewhat.