The article also addresses why this is not the chosen solution. It's pretty much the first one you'd think of: all kinds of existing software (that can be between client and server and out of their control) already handle GET bodies in all kinds of incompatible ways, because the existing standard says they're meaningless and "shouldn't" be included. The idea is to not break people's stuff, so they don't rugpull the established standard.
There's usually a reason why the simplest solution that pops into one's head is not "just" used by the people who put a lot more thought into it. Not always, but it can be useful to try to come up with it.
Arguably the only explanation you need is that "QUERY is the same as GET plus a body". The article just explains what GET is and isn't, but that can be implied.