The word interventional is not found anywhere in the article and it is hard for me to believe there are no interventional studies in this field.
> Multiple large observational studies have found that routine adult vaccines are associated with a reduced risk of dementia, with some showing risk reductions of 25% to 40%.
The observed difference is most likely caused by people with declining health being less likely to vaccinate. My assumption is that the article was compiled to show a conclusion that is not supported if one looks only a little bit further.
[0] https://www.science.org/content/blog-post/healthy-vaccinee-e...