I don't know, but whether they do or not shouldn't matter. They should design it so it can scale, 100k bookmarks is not really that big of a dataset for anyone, let alone G.

I don't see why they can't just fix this instead of asking him to revoke his plugin, especially if people are using it. It's needlessly removing a use case which may gain traction.

Of course they may know something I don't and which isn't mentioned in the article which makes doing any of this infeasible or undesirable.

Maybe they should just include the bookmarking in their search indices.

The bookmarks are already indexed. The pages that are bookmarked have a edge against pagerank for that specific user.

If I was a spammer or SEO, I would be figuring out every possible way - whether shady or not - to get someone to bookmark my page in Google Bookmarks.