The documentary speculates that dragons (if they really existed) could have contained large "flight-sacs" in addition to lungs, that stored hydrogen gas that may be produced by bacteria during digestion. This would give them lift (like a hot air balloon) making it easier to fly, given how big they are supposed to be, relative to their wingspan. It could also help explain how they breathe fire. If they had some kind of catalyst (like powdered platinum), then ejecting hydrogen gas + finely powdered platinum into room-temperature air would result in a reaction: ie. fire.

My thought was that the pteradactyls survived the dinosaur age. They descended into the dragons of the past, at least a physical form for them. Pteradactyles were large so the dragons could grow large too.

The flame would be a bit more difficult. I tend more to fine, volitile oils as the fuel for the flame. Starting it would be a matter of question. I am not sure there would be enzymes that would ignite the oil. They might gather flint like rocks in a pouch near the mouth and rattle them, possibly started as a mating thing, but that would spark the oils. Of course, how to protect themselves from their own heat would be a problem.
The hydrogen would be a problem due to permiability. I would think that it would escape the body all over.

Regarding the evolution, I believe that a small raptor, like microraptor (or a cousin), which is thought to have possibly climbed trees and to have had webbed armpits, like a flying squirrel, would be a better ancestor. This would better explain the tail and elongated neck in most dragons.

Another possibility could be an descendant of the plesiosaur, which could explain the "hydrogen sac".

In Native American myths, dragon-like creatures are associated with natural elements. The great horned serpent of Algonquian cultures, named mishi-Ginebig by Ojibwe peoples, is a representation of lightning. Invisible except when the brilliant gemstone above its eyes flashes with a light that blinds, Ginebig's depicted as a sky-tall, tail-walking giant serpent, walking on a bent forked tail. Forked horns reach to the heavens. Ginebig speaks from a forked tongue with the sound of thunder.

No wonder in early contact tableaus the True People thought the Strangers, with pale complexions like ancestor spirits, that came from under the world were god-like. They held lances that flashed brilliant light and spoke with Ginegbig's forked tongue and bit like the fatal touch of Ginebig. I suspect that the Western meaning of speaking with a forked tongue has a meaning different from the generally accepted one in some cultures. Rather than lies, the coercion of superior force.

I was discomfitted by Animal Planet's "What if Dragons Were Real" when I first saw it. Abstractions made concrete seemed a sacrilege of a rich history of human creative imagination corrupted by science for mass entertainment purposes. I sought a message or appeal underlying the premise to ease my anxiety. The eternal inner child longing to make sense of the unknown, and by making sense of it, own it. It's a present-day world of no absolute certainties, and the obsolescence of authority. Question everything. Challenge everything. Test everything. My inner child is saddened that fantasy must be made real.
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Hypergolic compounds ignite on contact. Like hydrazine and nitrogen tetroxide, liquids. They're toxic to life though, carcinogenic in low-dose exposures.

Triethylborane is a pyrophoric compound that ignites on contact with air, like white phosphorous, except it's a liquid.

With the dragon world we have created, the heat the dragons use is magically caused, but the flame, fire jells or other types of "flames" comes from the dragon itself. The way I am writing our stories, the dragons need heat to harden their shells to hatch. They absorb heat from the environment and then heat the shell. Parents build a fire around the egg to help, getting it hot enough. One dragon in our stories never gave up the heat absorbing characteristics and has become a frost dragon, taking the heat from the environment and even out of the "flame" and sending out a blast of frost. It fits the logic of the world we have created.