Yes. That is a pressure vent valve on the back side of the cap. When the tank builds up a small amount of pressure, it pushes the valve open and releases it. It is not a totally open vented cap. They are called "pressure vented". It is closed until pressure builds up and opens the valve and releases it.
You can verify this by putting your mouth over the valve and blowing. You can feel the valve release when too much pressure is built up.