If you put a sombrero on a cat, would that cat become Mexican? In short: no. The sombrero would remain Mexican and the cat non-Mexican. But the combination of the two would be considered problematic.
Accessibility overlays are problematic in a similar way. Or they would be if the cat had chronic constipation and the sombrero was on fire.
An accessibility overlay is a piece of third party software that is imposed (overlayed) on a website. There are many competing overlay products with names like:
Despite claims to the contrary, accessibility overlays do not and cannot make their underlying website accessible. Ultimately, they are just more stuff—and obstructive stuff at that. In fact, as the website overlayfactsheet.com attests, accessibility overlays have a habit of being inaccessible themselves, and can even prevent disabled people from using an otherwise accessible site. Which is a level of irony so high, it presents as
Even accessible accessibility overlays fail, by definition, and without exception. And why is that? Because, if the site is inaccessible with the overlay switched off, there's no guarantee the user will be able to switch it on. A situation you could be forgiven for describing as shiiiiiiiiite.
All successfully activated overlays provide is a perfunctory array of options for things like font size and contrast; things already provided by the underlying browser and operating system.
“But my overlay product isn’t just a toolbar. It also fixes your site’s existing accessibility issues, automatically.”
This is false.
Overlays cannot write you good alternative text, structure your document with an appropriate use of headings, connect the right labels to the right inputs, or determine if tables should have column headers, row headers, or both. They can't, and don't, design or build accessible websites.
Accessibility overlays are not so much snake oil as they are snake essential oil: a refinement of tech fuckery so pure and potent, they have achieved the Platonic ideal of shitty software.