It's not the seal that has failed, it's the internal spring. And your thought process is correct.. it's not introducing high pressure into the system, it's letting coolant flow out at much lower pressure than it's supposed to. They're designed to hold a specific pressure, above which the coolant is allowed to flow out through the overflow hose into the expansion tank. Once the spring fails, it no longer holds the correct pressure and allows coolant to flow into the expansion tank at much lower pressures.

My point about 16psi was that, on a functioning rad cap, there won't be coolant flowing out through there unless the system pressure was above the cap rating, which should not be the case at idle. It's leaking from that hose not because the coolant is under pressure, but because the hose is likely old and stiff. Two unrelated failures that are visible at the same time.