The author gives 3 arguments for not wanting to sign in at all. I find them all rather weak. The first two are actually a single argument.
The first argument: He likes to give consent. He fails to mention why he feels the need to consent to signing in, but the second argument implies it, making this just a prelude to that argument. This need seems irrational as there is no difference in the browsing experience between being signed in (and unsynced) vs being not signed in. The only difference is a minor UX change: you can now accidentally sync (which is his second, and most significant, argument).
His second argument, making it dangerously easy to sync, is actually a good thing for the average user. It is also not that simple to do by accident if you have a separate password set-up for syncing. I think it is safe to say on the whole that people who never sync will still not sync.
His third argument can be written off as paranoia: "For all we know, the new approach has privacy implications even if sync is off. ". Sure, maybe by simply being signed in you are being data-mined in secret, but if you trust Google that little why are you using their services? And what would be preventing them from mining you while not signed in?
Lastly, I think the author is slightly conflating "sync" with "data mining"? For all we know synced data is end-to-end encrypted and Google can't even read your bookmarks? The terms of service "signed-in mode" just states that data gets saved to the cloud, not that Google uses it?