I would be surprised if Google didn't know a given customer was disabled and serve up accessible results. Whether that is something an engineer deliberately coded or whether it was an algorithm that learned that a given cluster of users exhibits a strong preference for accessible sites.

For the SEO motivation to succeed, I think search engines would have to regress (by definition of "search engine" and its goals) to serve accessible results to users who don't need it. I'm not opposed to this as a solution, but it requires some interesting decisions: do governments stipulate how much of the search score for a given page is based on accessibility? Presumably it's a bad thing for governments to stipulate _how search engines work_. On the other hand, we could see search engines do this voluntarily; that would be cool and might just work because the search engine space isn't especially competitive, but if it ever becomes competitive, I don't think a gentleman's agreement to artificially boost accessible sites (at the expense of serving up the content that is genuinely most likely to satisfy a non-disabled person's query) is going to hold.

Note the distinction between accessibility and mobile--mobile was market-driven: lots and lots of Google users (as a percentage of total users and absolutely) search via mobile--it behooves Google to improve their search experience by boosting mobile-friendly results, and it therefore behooves sites to optimize accordingly. We're talking about boosting accessibility beyond its market value--I think this is good and right, but I wouldn't expect to solve the problem the same way as for mobile.