Not (entirely) convinced.

I reckon the session IDs they're referring to are the specific PHP session IDs that are part of the language (for cookie-free tracking), change for every (duh) session and expire. Consequently, they can't be replicated so that URL (effectively) never exists again ... so can't be indexed. Google are using ID as an abbreviation. not as a string to say any URL with "ID=" in it is going to be treated as a session ID.

This is substantially different from URLs which contain parameters the way they exist in dynamic Links pages (and many other apps).

You don't have to look far down any realistic search to see that "dynamic" URLS get well ranked. Yes, all other things being equal it's prob better to have a fully static URL, but that only holds true if you can conceive a mod rewrite project from the ground up to cater for all eventualities. Any leakage from a less-than-perfect mod rewrite implementation (resulting freaky destinations, 404s, etc) has to be offset against the SE benefits.

Maybe I'm just bitter because I've yet to find a whole-site mod rewrite solution =%)