General Elections are, at least nominally, fought on more than just one issue. So while I agree that the Lib Dems would have a mandate in the (more or less certainly impossible) case where they win an outright majority in the General Election, it wouldn't be unreasonable to feel uneasy about it. A referendum should really only be superseded (not subverted) by another referendum. Like with like seems a reasonable democratic principle.
As a matter of fact, I doubt that the Lib Dems would have the Legal power to revoke Brexit on "day one" anyway. At the very least, they would surely have to repeal the EU (Withdrawal) Act 2018, which is now primary legislation that (due to the Court ruling R (Miller) v SS Exiting the EU) can't be undermined by executive actions.
Of course, the Lib Dems would probably be able to push through the necessary legislation, which can just be modelled on the Notification Act 2017, very quickly indeed. The Lords would even be onside this time.
All of this is hypothetical. There is a slim chance at best that the LibDems will win a majority. I'd expect to see them increase their seat share, perhaps quite significantly, but no more than that.