It sounds like he is claiming that if a major browser changed this behavior, websites would just reconfigure their response to give a 200 response error page because they would prefer if the browser showed something like this rather than a browser dependent error message. That would end up making 404 completely useless, even for actual tools like wget because it would be against webmasters interests to use them anymore.
Well, my counterargument is that a change in browser behavior would make sites more usable, encouraging the proper use of 404. Of course, if that weren't the case, I'd agree there'd be no point in changing :P
Do you disagree with that claim?