Yes, that's right - instead of having to reserve a ticket or wait in long lines, he's already had a look around - showing off a photo of the singing Toads. Here's a rough machine translation of what he had to say, and spoiler alert... it sounds like he had a fun time as both a creator and player:
Masahiro Sakurai: "I went to the Nintendo Museum for a sneak preview of the museum. It was very interesting to see the history of the games that I have been involved with, both as a player and as a creator. I'm sure everyone has a different history and memories, but as for me, I just wanted to keep looking at it!"
Sakurai's most recent release was Super Smash Bros. Ultimate on the Switch in 2018, which was followed by two waves of DLC fighters and multiple amiibo releases. Sakurai also recently announced he had finished recording his final video for his YouTube channel and last December said he was still in the business of "creating games".