That's actually not a Triad rule. They're just following Dolby's guidelines and have been around almost since Atmos first came out.. Below is link to the April 2015 guide.
If possible, the height of the rear speakers should be the same as the height of the front speakers. If the room design makes this impractical, or impossible, the rear speakers may be higher than the front speakers. However, we suggest that the height of the rear speakers not be more than 1.25 times the height of the front speakers.
So in your typical room with a rear rise, do you align the tweeter on the front with the second row or front row? If you align it close to front row, there is no way you can get that ratio.
I am planning on putting my front speaker tweeters midway between 1st and 2nd row ear height, and rear speakers just high enough for unobstructed tweeter to ears of all rows. Don't really see any other reasonable compromise.