I may be wrong, but I doubt there are people using WebSQL storage as an in memory database that will be wiped out as soon as the page is closed.
It does open up some really interesting scenarios, though. For instance, I know a company that displays a large amount of data to it's users, so sticking that in WebSQL is useful to them as it makes that data easily queryable. However, since data is their thing they don't want it to persist!
I understand when this is done in a server, where you have control of how big your instance can be, but wouldn't that put too much memory pressure on the browser? What if the browser client is 32-bit? Even in Chrome where each page has a separate process one can make it go out of memory.
I think he was referring to the data manipulation capabilities of SQL, things like GROUP BY and WHERE and ORDER BY that can be offloaded to the client, not as a data caching benefit.
For a second I thought polyfill, as in polygon fill :)