If you joined the insider program before RTM and were activated (whether you had a underlying license or not) then you are activated. You can leave the insider program at any time and you are still activated. I've done this on both my MacBooks and many VMs.

What is interesting is that the OP managed to transfer this license as I thought it was only for the life of the device but then if MS are aiming for 1bn Windows 10 users they probably don't mind giving the few thousand early insiders it affects a free copy.

If you want to join the insider program now though you need to have an activated Windows 10 (which can either be Windows 10 key, an earlier activated Windows version/key or to have activated an earlier preview version on that device) so you are not able any more to get a permanent license just by joining the insider program.