I am not too sure but i think there is a hole in the top of the cap to act as a vent.maybe that is why those modle years had such a bad overheating problem,and maybe that is why chrysler dropped that style. I also think that was why they started having the coolant running through the heater core at all times. to help alleaviate the steam issue.You also have to remember that was shortly after chysler bought amc so alot of things was being experimented with and not everything worked as planned.On a brighter note it does tell you how tough the 4.0 is,constant overheating problems and still keeps on ticking over 200k when well maintained.

Another thing is he said he replaced just about the entire system so the bottle and cap would be new(he did mention that he replaced them) so even if it was pressurised that wouldn't be the issue.most likely it is a head gasket like you said or just garbage in the system that was left behind in the engine.