Deadlifts are a lot easier than you think in terms of technique. Grab bar, stand up. Gymnasts also resist multiple times bodyweight every time they do a landing, and they do so with incredibly rapid fiber recruitment. Look at the landing patterns... they tend to resemble a fairly straight-legged deadlift. Is it any surprise that they are capable of pretty decent lifts?

I'm not sure you are really familiar with the level of forces these kids get exposed to. They are also young, and have an overabundance of neural connections. It is much, much easier to learn and carry strength from one pursuit over to a new one as a child.

Gymnasts, actual gymnasts anyhow, spend considerable time resisting considerable forces with their spinal erectors and hands, and everything connecting their arms and shoulders to their bodies. Anyone halfway decent is going to be well-equipped to lift quite a lot of weight.

If someone can perform a planche push up with a straddle or full lay, they have immensely strong back muscles. There's no reason to think that they wouldn't have a fairly decent deadlift without training.

What we might think of the form, I don't know, I'm sure there's a wide variance there, but a reasonably well-rounded gymnast is going to be able to maintain an arched back with considerable resistance.

Remember that many of these stories come from smaller people, and the leverage + bar travel paths are also considerably less than a lot of taller lifters, so there is quite a large difference in the muscle you need to lift a given multiple of bodyweight.

For reference, it may help to keep in mind that for their weight classes that amount is still pretty low compared to the average competitor.

Much of this still holds true for a lot of bodyweight exercise enthusiasts over time, if you're being fairly well-rounded in your training.