I'm not sure what your point is. I qualified my claim very explicitly and what you said doesn't contradict any of it.

I'm not saying it's difficult to understand. I'm saying it's an unwieldy word and "a11y" is easier to remember and write correctly.

You specifically called it out as being "inaccessible" (ie, difficult to understand) to non-native speakers (of English).

Also, "a11y" looks too much like the English word "ally". That, IMO, is more likely to cause reading difficulties, particularly with non-native speakers and people with dyslexia.