Sublime seems to pretty definitively support only a single project per window based on the documentation available, although I could be wrong.
However it seems more likely that if you saw a window with project tabs at the top that it was something your OS was doing for you to meld the multiple windows together into a tabbed interface, which would mean that whatever it was is outside of Sublime’s control.
For example I think MacOS does something like that in more recent versions although I’m pretty sure in that case people have been complaining that it doesn’t seem to work properly because Sublime doesn’t cooperate with it. Presumably window manages for Linux or newer versions of Windows could do something similar.