This sounds like a likely date for it to launch on.
Super Mario Maker 2 was announced during the Nintendo Direct broadcast last month; surprisingly enough, the release window announced for it wasn’t late this year or next year, it was June. As in, under three months from now.
Now, we may have some idea of when exactly in June the game is launching. According to what GameXplain is reporting, Amazon UK has started to email customers who pre-ordered the game on their site that their orders will be shipped on June 14, indicating that that is the date we will see the game launch. June 14 is a Friday, for what it’s worth, so a release on that day does make sense and fall in line with Nintendo’s release patterns. It would be right after E3 as well, which I would imagine Nintendo would want to avoid, but Mario as a brand is big enough to avoid getting overshadowed.
Remember, this is only a rumor for now, there is no official confirmation. Still, as mentioned, it seems likely enough to be true.