I'm pretty sure that's not how it works There is only 15 psi in the entire system when the temperature is quite high. At 180 degrees there is a few psi on the output side of the water pump (that's what moves the coolant through the radiator, heads, block...) but the inlet side can very well be under suction. A lower hose collapses because it's being sucked in, which by definition means there is negative pressure inside it.

Now if there's a restriction in a clogged-up radiator, it's even more likely that the water pump will have vacuum at its inlet (the lower hose)...