alt Hacker NewsI don't think that's the same, is it? That's just Spotify letting you play files on your local device. With YT Music you upload them from one device to the YT Music cloud, and then any device can access them/stream from the YT Cloud. You can't upload your music to Spotify, only use it to access media that's stored locally on your device.
Last time I tried, I was able to load the local file on desktop, and then sync that playlist on mobile (to listen to the "desktop song" on mobile even when offline). So I'd argue it provided similar functionality.