I agree. Dive bombers are generally a more vulnerable than level bombers because of their tactical employment and limited size. We should look into these rather than looking into armament - in that regard, a Stuka or any other dive bomber was not really worse than say an early He 111. Both could generally point a single machinegun against an attacking fighter, which meant they were greatly outmatched.
Of course level bombers had more guns covering most attack angles, which could support the other bomber when flying in close formation. Dive bombers usually had guns firing to the rear, as turrets would be grossly unecomincal in their relatively small airframe (compared to a medium or heavy bomber's. Keep in mind that the Stuka is a rather sizeable aircraft still, for a single engine type!).
What was really important IMHO was how the DBs operated and attack. Stukas, especially later models were in fact quite capable of fending off fighters, especially single fighter attacks when flying in a mutually supportive formation. After the diving attack, the formation was always scattered, and took some time to reform. During that time, each dive bomber was on it's own, and usually the heavy losses were suffered if enemy fighters jumped them during this period - this happened a couple of times during the Battle of Britain, for example. In contrast, level bombers kept supporting each other during and after their bombing run, making them much less vulnerable, and of course their larger airframe could soak up a lot more battle damage.
The diving attack also made them relatively easy targets for light AAA guns, if present, as they flew close and on a predictable path.