Microsoft released a app which makes Windows 8 check for updates when the TP is released a couple of weeks back. You may have ran this app which is why you've gotten this update.
I recall reading a few weeks ago that Microsoft was planning to release a Win10 preview *upgrade* through 8.1 wupdate. You must have found it... I'd recommend hiding it and forgetting it, myself. If you want to upgrade or clean install, download the .iso, instead...
So is it a yes or a no?! Does anyone have a link to this app? Would be awesome to "move" some Windows 7 Home Basic folk to Win 10
He hit the upgrade button on the official windows 10TP website and ended it premature but the upgrade is now offered by ms update. Every normal thinking person would not install/upgrade to a preview (with ms looking over your shoulder) instead of a fully functional windows (vista, 7 or 8.1)