It does not matter that the leapers don't promote because pieces are never taken out of the game forever, they all return via drops. The stronger player always has enough pieces to checkmate.
What matters is that many of the leapers are so weak as to be ineffectual against a piece like a King without the possibility of promoting to something stronger. Even the weakest leapers in Shogi, the (Shogi) Knight and Pawn, have some way to work around that with their promotions to the equivalent of a Gold General. But I suppose it is good enough as a proof of concept.
What matters is that many of the leapers are so weak as to be ineffectual against a piece like a King without the possibility of promoting to something stronger. Even the weakest leapers in Shogi, the (Shogi) Knight and Pawn, have some way to work around that with their promotions to the equivalent of a Gold General. But I suppose it is good enough as a proof of concept.