Stores leaking unannounced games and release dates is nothing uncommon, and even though they are often right, they are just as often wrong. There is a general rule of thumb you can use, and that is that a bigger chain of stores is more likely to have the right intel than an independent smaller outlet (at least that's the case for most of these stories)
Right now, no store is bigger than Amazon, and the Spanish branch has published a release date for the upcoming Mario Tennis Aces, which is released exclusively for Nintendo Switch. According to the pre-order page, the game launches on June 22, which fittingly enough is a Friday. It also makes sense in relation to Nintendo's other big Switch releases during the quarter.
We still wouldn't bet money on this, but at least it's a way better leak than many others regarding these things and it could very well prove to be correct.
Are you excited about Mario Tennis Aces?