Once a modicum of thought is surpassed, perspective begins to shift.
Or someone thinks they're putting in a modicum of thought while in reality they're doing mental gymnastics (Without actually providing any substance to a viewpoint other than "Hurr durr dumb person is dumb" which seems to be your modus operandi for responding on these formus).
Dragonborn aren't dragons. That's a fact. It's not "Flavour" for Dragonborn to have dragon abilities because they're not dragons.
They're descended from dragons and have some dragonic elements such as the reptillian appearance, resistance to their element type and the ability to use a breath attack. But that is as far as it extends. There is no mention of them having dragon scales just as there's no mention of them having dragon wings.
Without dragon scales, there is no reason for them to have any bonus AC.
Could they be retconned into having dragon scales? Sure, but you'd also have to add some additional mental gymnastics to explain why they get ONLY 13 AC instead of an adult dragon's 19 AC... Or maybe you just give them baseline 19 AC and break the balance of the races with such an obscene bonus just because you can't comprehend anything besides your random headcanon "Flavour"