This doesn't even have to be a "rumor" or a "leak". It might as well be educated guessing.
Nintendo has had very little Metroid presence on Switch so far, but the eShop HAS been hosting a lot of indie games inspired by Super Metroid.
If they want to sell something as absurdly titled as "Metroid Prime 4" to a host of Switch owners who have (presumably) no idea what that means, they better bring out the best Metroid stuff they have in advance of that release. And play up how big an influence on the indie movement Super Metroid has always been. Cave Story is not NOT influenced by Super Metroid.