I see where you & cudavert are coming from, but something is causing enough pressure to make that hose swell up like that. If it's pressure just from boiling then it's got to be pushing more than 13psi also. So if it's not pushing water from that why wouldn't it not push water from a bad head gasket too? Pressure is pressure right? It takes more than 13 psi to swell a hose like that. I have seen many gaskets leaking between the combustion chamber seal into the water jacket & barely push, if any water out of the overflow. The combustion gases sometimes swell the seal in the cap so it can't bleed the water pressure off correctly or the engine pumps pressure in so quickly it can't bleed it off. It also softens the hoses real quick.
I'm just spit balling ideas here as well. Not just in this case but if others may run into an overheating problem down the road. Some things to consider.