I don't believe it. If there were a mastery to this, it would be shared. All that's happening is they are cutting the bottom off and sealing. If I'm wrong show me a method for one of the simpler designs.
I think the simplest design would be the classic, literal, 'ship model built in a bottle'. And with that, it's literally all about manual dexterity in gluing together a model of a boat with all the parts glued on from the end of long tweezers and similar tools. In that case there's nothing 'impossible', it's just an incredibly tedious and painstaking process, and that's the only point of it.
It’s like pulling teeth through the anus. Theoretically possible, but very, very difficult.
The page literally has a step by step guide for one of the simpler designs.