If the cap is vented I would agree with Steve.
Our tank is vented at the tank - terrible idea - when we fill it, and then are heeled over for a while we get a floor full of water
. Only 75% of our capacity is useful. I suspect someone changed things on us some time ago.
On your system, if this is the vent, the top is well above any possibility of spilling water while heeled.
Also if the tank fills from the bottom, it has to be vented to let the water in.
The only thing I am curious about is what happens when water enters the top of the stand pipe - maybe there is an inverted J inside at the top with a floating ball valve - like some snorkels, that stops water from entering the stand pipe as you fill.