There's that cavalry advice in your link bsms
I know. Apart from that, it has good advice. An Internet falsehood, once started, never goes away. After all, it can always be found on the Internet...darn near forever...
20% as a rule of thumb is actually counterproductive. It is the total weight the legs need to carry versus the bone structure in the leg to support that weight that matters. There are a lot of big QHs with slender legs who are hard pressed to support their own weight, let alone another 20%. OTOH, our mustang pony Cowboy is 13 hands and has the legs of a 16 hand horse. He has plenty of reserve strength built into his structure. A heavier rider needs a horse with overbuilt legs, not a heavier horse.
A horse who becomes unrideable earlier in life almost always does so due to leg problems, not back problems. I don't know anyone who has stopped riding due to their horse having back problems, but quite a few that stopped riding a horse due to its leg problems.
The back problems I know of have been due to too much weight distributed over too small an area of the back muscles, causing damage to the muscles. The solution was to get a better fitting saddle to protect the muscles.
Cowboy carrying my 5'8" and 180 lbs: