My opponent didn’t set full defense

No, this should be an easy win for you. They probably just forgot or didn't realize how many teams each zone needed.

The only strategy behind not setting full defense is taking the L

Ok thanks.

Some put a single team up & inentionally leave the remainder empty.
A rude way of admitting defeat if they feel your roster is overpowering to theirs.
The opponent intentionally denies you some feat progression as an added twist.

Edit: I'm Division 3 and only seen it happen to me once or twice.
I've heard it's become more common for those without GL toons to be doing this to those with GL at higher Divisions.