I quickly skimmed the article. I’m sure that ANSI and the ICC both want to be recognized as safe harbor just on principle of promoting their work. That said, if the requirements are substantially equivalent to FHA when a jurisdiction adopts ANSI, it certainly makes life easier for the designer to track just one regulation.
I also think FHADM is frozen in time and is unlikely to be updated anytime soon. While no one wants to adopt 2017 ANSI 117.1 due to the 67” turning circle, 2009 is an OK place to land. If ANSI wants newer versions to be relevant, they may want to revisit that 2017 requirement.
FHADM also has problematic elements. It requires EXACTLY 18” from centerline of toilet to side walls in Type A units. California code has a range of 17-18”. So code+regulatory compliance demands installation perfection, zero tolerances. Some CASps will say 17.75-18”, but very hard to achieve, especially at slab-on-grade toilet rough-ins.