Of course, adding 3D colliders will be a significant effort even if you consider it could be an extension to the existing 2D colliders. Also, at some point it no longer make sense to use a 2D editing software if your level requires 3D colliders. As you can already see in the video you linked, it becomes very difficult (or even impossible) to see where a 3D collider is actually positioned - you need to look at the numbers in the properties view for that.
That said, I’m all for extending Tiled’s support for objects to support 3 axis rather than 2, and along with that to also support 3D primitives. But especially for the latter I’ll need to see a significant demand (in comparison to other requested features), before I can hope to get around to it.
Btw, this seems like quite a basic extension to Unity and it’s still costing $40… Seems overpriced to me, but judging by the reviews it’s worth it. I wonder what I should be charging for Tiled then.