Skype is a Microsoft service and uses Microsoft accounts for authentication. Skype for Windows 8.1 (the full-screen “modern” interface version) may have required you to use a Microsoft account for Windows login. But that’s a moot point now because Skype for Windows 8.1 has been discontinued. Windows 8.1 users are now instructed to use the windows desktop version of Skype. (Which has always been an option, and which many people already did.)