Yep. The rule pretty much constrains your options to current standard practice. No showing up to a race with handlebar bottle cages, like in the "heroic" era of road cycling!
Having braze-on bottle cage bosses on both downtube and seattube as standard is something that really only became standard in the last few decades. Most road bikes circa 1980 had cage bosses on the downtube but nothing on the seat tube.

Not permitting bottles on the saddle is not surprising at all. Besides the aesthetic consideration, a lot of people do a poor job of setting up saddle cages. Very few people I ride with use behind-the-saddle cages, but most of the bottle launches I've seen on road rides have been from that location. It can be done right, but you need to position them properly and use cages with good retention. A lot of people just don't. In a tight group, that's dangerous. Roadies are a very traditional group, and if people are doing a new thing that's dangerous when done poorly, and they're doing it poorly, expect that thing to be banned.