Only real problem with that idea is that for alternate timelines, you need Time Travel, and with the exception of Whis (who has a 3 minute limit), there is no time travel after the Cell Saga, therefore no possible point for the timeline to split.
Another problem: In GT, Pilaf is really old, but in SUper he's a kid again, having wished for youth with the dragonballs. Even if we don't know how fast he ages, we do know how fast Mai ages, since she's human, and therefore she can't have been made young again at any point in GT. Unless the timeline is split by Pilaf's wish for youth, they cannot co exist. But why would anyone use time travel to prevent Pilaf's wish for youth, when there is so much else going to sh*t in super.
Also, yeah, it's not an alternate timeline. It's just non canon.