I'm not sure if the word "dragon" has a precise meaning.
It has many meanings. Some of them vague. Some of them precise. Different cultures, mythologies, fictional universes, etc. have different ideas of what properties dragons have. There's no one precise meaning. In context, it might have a well defined meaning, such as if we're discussing the properties of dragons in the Tolkien's legendarium.
But we can discuss dragons as a general concept without me, you and Jack by the Hedge ever actually agreeing on a definition. Which is exactly what we're doing.
I don't know if the word "god" has a precise meaning. But I am sure that if we want to talk about dragons or gods we will have to agree on how we use that word"
Do we? I can discuss religion with people, tell them I don't believe in God, and we both can agree that I am in fact an atheist and don't believe in whatever god they do, without us agreeing to the particulars of how they define their God.
I'm going to assume unless they say otherwise, that their god falls under the general concept that people have of god in general, i.e. some sort of personal super powerful being that created the universe, or something along those lines, in which case I don't believe it, without having to discuss with them the precise properties of their god. I don't need to know whether or not their god answers prayers, or is concerned with people's hairstyles, or sacrificed themselves to themselves for silly reasons, or whatever, to know I don't believe it. If they use the word god somewhat idiosyncratically, then it's up to them to explain what they mean, but I generally don't care, because as soon as someone starts describing their own weird idiosyncratic god beliefs, I immediately think "sounds daft, I don't buy it." A lot of the time you simply don't need to get into the finer details of someone's beliefs to know you don't believe it. I'm also not particularly motivated to debate or give serious thought to everyone variation on god beliefs out there, anymore than I need to listen to Scorpion's odd ideas about how reincarnation works, to continue not believing in gods or reincarnation as general concepts.
Evidently, Jack didn't want to argue about the existence of dragons. Did you?
Of course not. It was an analogy to make a point about discussing the properties of things that don't actually exist. You don't need to agree upon an exact definition of dragons or gods to be able to dismiss them as a general concept and to therefore dismiss anything that falls under that general concept, i.e. people's idiosyncratic ideas of what god really is to them.
If you pretend to know that X is silly without knowing well what is it you are a dogmatic.
You just said that you don't know what dragon precisely means. So you don't "know well" what is meant by dragon. So it must be dogmatic of you to say dragons are a silly idea that doesn't exist? Are you agnostic about dragons?
Do you think I'm dogmatic because I've dismissed the idea of dragons as silly, without getting into the particulars of how any given person might define the word dragon? The same goes for god, regardless of whether the people defining it, take it seriously.
Don't bother pointing out that people (or at least very few people, I've seen cryptozoologists online claiming dragons exist) don't believe in dragons but lots do believe in god. I know that. That doesn't negate the analogy.