Does it make sense that there are people who want to come in from moment one and have the full suite of tools available, and not having that ability is annoying? Yeah, sure. But the level of impatience comes off as just a little bit weird, as though if you had everything at the start, 9 days would be all you need to make all the levels you'd ever make, anyway. Not to mention the game comes packed with some, what, 60+ levels to already play through, and even with day-1 tools I'm sure there will be some really interesting stuff done online with it. This isn't the sort of game that's going to be given up on after 9 days like other titles, where you can blow through a campaign in a few hours, try out some multiplayer, and call it a day.
It's not exactly new news that Nintendo is very careful about how they craft people's experience heading into a game of theirs, particularly in teaching mechanics for a game style that they've never really done before. It happened with Splatoon's slow-drip release because of the assumption that a lot of the game's player-base wouldn't necessarily be familiar with online shooters, and despite some internet grumbling, that seems to have gone well for them.
Would I personally prefer to have everything available at the start? Yeah probably, but I'm not heading to the hardware store for a pitchfork over nine days for a game that will likely be played for years. Getting a small package of new items each day encourages builders to test out levels with each of those new features rather than running for whatever their one personal favorite might be. Maybe they don't want to overwhelm people, or maybe they just want to make sure that even the most basic features are given proper time, attention, and utilization. Either way, the point will be moot within two weeks of launch.