When you actually look at those "promising results" you'll notice that they come about by cherry-picking, not controlling for confounders (like Esperanto teachers using better teaching methods) and wishful thinking. See https://news.ycombinator.com/item?id=14848019 , which I wrote some time ago when the same topic came up.

I don't doubt that Esperanto is easy to learn, but there's no evidence to suggest that learning Esperanto helps more with learning another language than spending the same time learning that language instead.