Yet people do make claims such as 'I believe that there are no dragons' all of the time. Either:
a) those people fit the definition of delusional (as per your argument)
b) The statement of the form 'I believe that there is/are no x is invalid, and those making those claims are equivocating it to the statement 'I lack belief that there is/are no x.
(a) seems to include the entirety of humanity.
Does x = god not apply to this case? Are there other options aside from (a) and (b), or are those options invalid?
There is at least one big difference here: People are generally agreed on what a "dragon" is. There would be some variations in details, but most, if asked to describe a dragon, would say that it is a winged creature, vaguely reptilian, significantly larger than a human being, with a tail and four legs ending in claws, that has scales and that lays eggs, and which is capable of breathing fire. They are also generally assumed to be tremendously strong -- for example, they are routinely portrayed as being able to pick up and carry full-sized human beings without showing any effort or strain.
Knowing all of this, one can then examine nature and determine whether such a creature exists, or is even likely to.
For example, are there any flying creatures capable of carrying a grown man? No, not even close. The Harpy Eagle is arguably the strongest known flying creature in this regard, and even it is generally capable of carrying only about twenty pounds. There are no creatures capable of breathing fire, and it's difficult to imagine how this would even be possible. (Two glands in the throat containing different chemicals that would combust when mixed, or something?) There appear to be limits on how large an animal can be and still be able to fly, and the generally-assumed size of a dragon is significantly beyond that limit. Ostriches, for example, lost their ability to fly because they simply became too heavy to be able to do so, and dragons are usually held to be far more massive than ostriches. And so on. All that being the case, it is not therefore delusional to claim to know, with certainty, that dragons do not exist. There are good reasons to hold that position. (It would
be delusional to continue to cling to that position if new information were to come to light, of course, but that is a separate discussion.)
None of this is generally the case with the Christian deity, because no one agrees on its characteristics. Some are trinitarians, some are unitarians, some are even binitarians, as I learned just yesterday. Some believe that the deity is omnipotent, others do not; same for omniscience. Some believe that Yahweh is a some kind of spirit-being, while others believe that he is flesh-and-blood. Still others embrace a form of pantheism. There is also disagreement about Jesus' characteristics. One of the earliest schisms in the church, for example, was disagreement over whether Jesus existed as flesh-and-blood, or whether he existed solely as some kind of spirit-being. On and on it goes.
It is therefore more difficult to make a case for "Strong Atheism" than it is for "Strong A-Dragonism", at least in the case of the Abrahamic religions. In other traditions -- such as the Greek or Norse pantheons -- it does become more straightforward to make a knowledge claim, because those deities are more precisely defined. "They live on the summit of Mount Olympus? Fine, let's climb it and look for them."