From OP: Interpretation ?

No, that is not correct. It all depends upon whether that stand alone tank is an expansion or an overflow tank The former is pressurized, the latter is not. Read this:

The OP stated that the vehicle is a 2005 Honda Accord. That vehicle has a common overflow tank. Not a pressurized tank.

Correct. This is a non pressure tank unit. Normal operation is:

Cold start with a full system and full radiator (IE you topped it off) As the engine warms up the coolant expands until it reaches the pressure the cap is rated at. When it hits that pressure the valve in the lower section of the cap gets pushed up and the coolant goes through the neck of the radiator into the overflow tube and into the reserve/overflow/capture tank. (This is different than earlier vehicles that just dumped the excess onto the ground). Now the coolant gets to a point where the pressure in the system stabilizes as the temperature stabilizes. At that point the overflow should be checked. It should be on the full hot mark. If not you top it up.

Now as the engine cools off the coolant starts to contract as well. This causes the system to form a vacuum. The vacuum will do one of two things. On some vehicles it will simply draw air into the radiator through the valve in the center of the cap. OR on many vehicles the vacuum will draw coolant out of the overflow bottle and keeps the system from drawing in air (this is common on most newer vehicles because air can cause problems when mixed into the coolant)

The accord uses a gravity drop into the overflow. Unless the tube is blocked the coolant should go into the bottle either way.

Which makes no aspect of my above comment incorrect. Before even delving into the subject of an exterior tank, I clearly stated "I'm not certain as to the application's system but he clearly stated radiator cap, not coolant recovery tank". And in any case, if he's referring to a radiator cap replacement, that cap is either on the radiator itself, or an external expansion tank. There is nothing on a strictly overflow (not expansion) tank that could remotely be confused with a radiator style cap.

Or if there's a leak in the system, what ever medium (e.g., air) is adjacent to that leak may be drawn into the cooling system, possibly providing an erroneous indication (from the appearance of the level in the overflow tank) that the coolant level in the system is nominal, despite the fact that it can now be low.

Fluid forced out of the radiator by expansion of the coolant is fed from the cap area, through the overflow line, and into the bottom of the partially filled OT, thereby always ensuring that the system remains free of air. Gravity maintains fluid at the bottom of the tank, but neither the filling of the coolant into the tank (pushed out of the radiator by fluid expansion) nor its draining out (pushed into the radiator by air pressure as a result of a partial vacuum created by fluid contraction) is "gravity fed", per se, unless of course one considers that rho*g*h is a representation of atmospheric pressure.

Depends what "defective radiator cap" means.

The cap has several functions. It acts like a pressure regulator that maintains the cooling system design pressure. It also acts to divert excess fluid and pressure into the overflow and allow the fluid to return when the pressure drops below normal atmosphere pressure. There are several gaskets (rubber seals), valves and springs which could be defective.

It's possible the cap itself is allowing fluid to leak out of the system when you aren't looking. Or if the system is running with too high or too low pressure that also could result in leakage that you don't notice.

The easiest way to check the cap is put a new one on and see if anything changes.

-jim