it's not entirely clear from the page - the description of what you get when you buy the "dev kit" says

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Each kit includes a custom Playtile and a download key for Gumpy Function's enhanced version of "Let's Build a Platformer!" course, featuring exclusive Playtiles chapters on sideloading, testing your games, digital manual, achievements, and more.

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so at the very least it sounds like you buy the dev kit and it comes with the device as well, and the PDF course book. but it's not clear whether you can just get a playtile and then develop your own games for it. they do say you can "sideload" games from itch.io and play them with an actual controller, which makes me think the sdk is very minimal and mostly involves input translation, and might well be documented on their site when they release. it would be nice to have that spelt out in the faq though.