Gameplay and Lore are not the same thing, at least not in my book. In Lore, a Dragon is the kind of thing that can devastate an army in moments, obviously they can't be that strong in the game, or you would never be able to defeat them. As they are they're generally too easy to kill, which is a pretty massive inconsistensy with how they've been described in the Lore. Even if you disregard that, a dragon isn't really dead until the Dovahkiin eats it's soul. Even if some town guards could kill a dragon attacking their city, so what? Alduin could swoop on in and ressurect that Dragon whenever he wanted to, or just kill everyone given that he is literally unstoppable unless a demigod travels into the realm of an actual God and kills him there.
Yes, there is a difference between the actual game and whatever fantasy lore you believe. However, your wishful thoughts about lore are just your imaginative flights of fantasy and pretty much meaningless. If it does not exist in the game, then it doesn't exist - at least in my book.
To me, lore (and especially from some web site) is a crutch for the mindless and those incapable of reasoning. Why anyone would believe something that is formulated by someone's comment on another's thoughts regarding some persons assumption about a speculation made on some other persons idea on an individual's thoughts providing a fantasy explanation for some game aspect is beyond me - but please feel free to believe such ilk, just don't insist I must believe it too.
While some may choose to believe information born from fantasy, I prefer to formulate my own thoughts from information that actually exists within the game.
There is no powerful dragon just over the horizon killing an entire army in one fiery (or frosty) breath that I cannot see.
Dragons are easily defeated by anyone with some fighting skills and are not all that powerful.
Alduin is not unstoppable and all it took was an ordinary mortal, assisted by immortals, to defeat him.
While these may not suit your fantasies about the game, I am not insisting you must accept them. So, please, feel free to wallow in whatever imaginative fantasy you want, but don't tell me I must accept your fantasy as my thoughts of the game.