Try the cold engine hard coolant hose test yourself. After the car has sat overnight the coolant hose should not be hard and under pressure. I know the test sounds strange but is more than a part of fokelore on your engine the 603 type.

I would preffer the other ideal of releasing the cap for a second on the overflow chamber to release any contained pressure. Thats when the engine is cold and really there should be little or no presure present. Start the car cold. Check to see if the hose is pressurizing or going hard fairly fast.
it should not.

The water pump is not circulating until probably 175-190 degrees is reached. So any hardness should be from a cracked head or head gasket. In fact it is prefferable to do both tests in my opinion. Yours hopefully will pass both tests.

Never loosen that collant cap with a hot engine. Especially if abnormal pressure might be present. Some pressure is going to be there by design. This applies to any car. The scalding burns could be pretty bad.