I found the solution, still not sure why this works and my previous code didn't, but there you go:
models.Playlist.createTemporary(playlist_name).done(function(playlist) {
playlist.load('tracks').done(function(loadedPlaylist) {
loadedPlaylist.tracks.add(models.Track.fromURI("spotify:track:4VqPOruhp5EdPBeR92t6lQ"));
loadedPlaylist.tracks.add(models.Track.fromURI("spotify:track:5HF5PRNJ8KGtbzNPPc93tG"));
});
var list = List.forPlaylist(playlist);
document.getElementById('playlistContainer').appendChild(list.node);
list.init();
});