It is true that a lot of mexican food can be vegan, and even there are some very good vegan options, in practice most of the recipes are not vegan by default. Red rice and fried beans are often prepared with lard, enchiladas and chilaquiles almost unvariably have cheese and most have egg, beef or chicken. Most common tacos are not vegan obviously. Tortilla soup and fideo (pasta) soup use a chicken broth base.

Obviously there are great alternatives/adaptations to these dishes but you can't expect them by default if you eat out. That makes me seriously question the ~15% claim...

Actually, looking at online sources it seems that it could be closer to 3% - 9% (tho personally I think it's closer to the 3% than 9%).

https://es.statista.com/estadisticas/1425462/edad-de-la-poblacion-vegana-en-mexico/ https://www.eluniversal.com.mx/nacion/mexico-encabeza-la-lista-de-veganos-y-vegetarianos-en-latinoamerica/ https://noro.mx/es-mexico-el-pais-mas-vegano-de-latinoamerica/ https://en.m.wikipedia.org/wiki/Vegetarianism_by_country

Edit: Apparently OP image comes from the wiki link I listed, where the table shows 9% instead of what I assumed ~15% based on the image alone. I guess that checks out... still feels a bit high but could be maybe. I guess I should look at the sources more carefully.