It looks like Splatoon 2’s release date has been leaked by GAME in the UK. Now it’s quite often that placeholders are added to listings but for these listings the date here is very specific, and on a Friday too which is common for new UK releases.
Here is an image of the listing on GAME.
Splatoon 2 was only given a release window of Summer 2017 so again that date fits the release window too so it could be the actual release date. It also helps that GAME are the biggest video game store in the UK so it’s reputable source.