I, too, would be rather surprised that incognito/private-browsing would share cached data (and thus ETags as sent on If-None-Match requests) with normal browsing.

Looking at the researchers' paper...

http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1898390

...it's not clear that's what they're claiming. One quote is that "Even in private browsing mode, ETags can track the user during a browser session."

That suggests they may be concerned about cross-site tracking within a single private session, and the possible expectation that 'private browsing' prevents tracking from site to site. (I've never had that expectation; only that a private session is not connected to distinct prior private and non-private sessions.)