The reason why an advisory?
Well, after market under seal is not standard equipment but bear in mind the nominated tester only has an inspection lamp , prodding tool and pry bars to use. They are not permitted to remove any part fitted to a vehicle to inspect behind or damage any painted area, so if they can't inspect the must offer an advisory on the item. Even painted and covered brake pipes are subject to a small bore tool with a rubber ring that tests the thickness of the pipe without scraping off the protection. If they see corrosion, they are permitted to tap with the prodding tool plastic handle and then prod and scrape the area with the blunt alloy end only. But they can't use excessive force to push through and do further damage.
So if it under sealed, it's an unknown quantity that is not available to inspect where it normally would be as standard on the car as normal, thus the advisory notice informing you.
Here is a tip for the future.
After the under seal has dried get hold of a cheap spray tin close enough to the original car colour and give the floor a quick spray over with that, yet another protective coat giving over the under seal... They won't even bother checking after the first tap and prod as it looks standard to the original vehicle
Hope this helps