Well...it does sound like a case of using a poorly fitting slot screwdriver but only you would know that for certain.
I don't know this outboard but if there is enough clearance and the screw head is protruding, you can get a Dremel tool with a cutting wheel in there and cut a new slot 90 degrees to the old one. It takes a steady hand but that's how I fix that sort of "screw up" all the time.
If the screw head is recessed in the housing....that's a tough one. I use a set of cobalt left handed twist drills to remove screws like this. As the drill edge "bites" it typically will unscrew the screw while I drill. On some though...a screw extractor must be used. They can be a real pain.
Barring any that...it may be necessary to take the tank off and deal with it. And doing that...on an outboard that is under warranty....might best be left to the Honda shop. Even if they charge you a few bucks to do it.
I do know this: To keep from messing up the slot in pretty much ANY screw head you MUST use a screwdriver that is almost an EXACT fit to the slot. Especially the brass/bronze screws they use with the fuel systems. If you're not paying attention to that detail then it will bite you again.
Good luck.