I had the same misunderstanding as well. However, I think they ought keep Windows 10 Insider program available to everyone free of cost and without having to pay for a license. I feel like this will fall on deaf ears though. Microsoft watchers say Windows already treats stable as a testbed for the enterprise users who withhold updates for n days. So, a new update shows up without much testing, breaks a bunch of stuff on consumer hardware, Microsoft finds out thanks to logs or twitter chatter and fixes it, and everyone including enterprise hardware take the updates.
I think eventually Windows being source available is possible but I doubt it will be free in a meaningful way.
So was Microsoft Office when they were trying to put Lotus, Wordperfect, and every other word processor and spreadsheet out of business. And they succeeded. Follow the pied piper boys and girls...
It was a time limited free upgrade for people who already had a valid windows license. I don't think it it was free.