You're arguing in circles. JandK is saying the mechanics and flavor as written don't make sense ludonarratively
Based on nothing besides the word "Dragon" in the name of the race.
You're coming up with reasons those scales wouldn't give armor and the others would, but it's based only on "bc that's how it works right now".
Based on the actual lore of the race.
The lore of the race does not specify that they have the same scales as dragons.
The lore of the race does not grant them all of the features of dragons. With what features they do have being significantly weakened due to their genetical distance from their dragon ancestors.
This is backed up by the mechanics also not providing them with all of the features of dragons.
Flavourwise, it is not necessary for them to have an AC bonus as nothing explicitly mentions in their lore that they have protective scales.
Also yes snake scales are more protective than just skin. That's how scales work.
Scales ARE skin. The only major advantage they have over skin is better retention of water to prevent dehydration.
Scales are no more resilient to damage than skin and in fact most of the tougher animals simply have skin, but very thick, things like rhinos, crocodiles, whales. With exceptions like pangolins whom have tough scales to provide additional protection as well as some deep sea fish that have hardened scales (Or things like sturgeon that reinforce their scales with bone scutes)
Which brings me back to my point that scales are not all the same. Some scales are specifically hard like those of the pangolin or dragons. Some scales are soft like those of snakes and apparently Dragonborn.
Simply having scales is not sufficient excuse for having bonus AC. Unless lore is written that Dragonborn specifically have hardened scales that offer additional protection, then it is not out of flavour for them to not receive bonus AC as a racial feature.
Outside of such lore, then it is not a forgone conclusion that Dragonborn must have bonus AC due to being scaled.
