I am not quite sure why an extra super would help prevent swarming if you empty your flow frames when they are full and capped.
I have added an ideal foundationless super (above flow box) at some stage to give them more space and work, but it only slowed down the ff filling for a couple of weeks. Not sure the additional ideal was necessary, but it made me feel better, because I thought I had too many bees out there washboarding and boarding. Which by the way stopped after adding the ideal.
If he meant another brood box, it would make more sense, but even there would be other strategies one could employ in our climate.
I take out a brood frame once in a while to give to another of my hives and replace with a foundationless frame. That way my comb is always freshly built and pristine.
This worked very well for my first season as a beekeeper, but I am sure there are plenty of other strategies around.
Early spring I will split my strongest hives and intend to manage the season as above.