"eliminating benefit of vegetables"

That's a pretty wild jump.

I’m not sure from skimming the page—did they explicitly give different groups different diets, or did they just measure their ergothioneine levels and conclude that they must have eaten more mushrooms?

Could it just be that dementia-prone people have differences in their metabolisms that cause them to process ergothioneine differently?

Genuinely, how much can we trust a US government website on anything claimed to be health information given who their health secretary is?

nih.gov

The title of this post exaggerates the conclusions of the study. The study shows a negative correlation between ergothioneine and dementia which is independent of other factors such as cardiovascular health and vegetable intake. This factor being independent of vegetable intake is not "eliminating benefit of vegetables".

Also, while mushrooms are the food with the most ergothioneine by weight, adequate dietary amounts can be obtained in other foods - oats, some beans, garlic, ginseng etc.

It seems to be good research with solid conclusions about ergothioneine intake and dementia but the headline is misleading.