So who actually came up with the octatrack then?

Well, you can read the credits for each instrument in the manual, and a little internet searching will find you a lot of information about how the SidStation grew out of a student project, but here’s something from the old Elektron Users forum (the only post ever by Jesper Kouthoofd of Teenage Engineering ) from 2010 describing the creation of the OP-1, which also talks about the earliest concept of the Octatrack:
https://www.elektron-users.com/index.php?option=com_fireboard&Itemid=28&func=view&id=95826&catid=13&limit=9&limitstart=41

Here’s a very quick Elektron history:

and something from around the time of the OT’s introduction:

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