The "reversability" of Art. 50 has come up because it is a point of law that needs to be settled in the current legal proceedings against the government arguing that Art.50 must be triggered under an act of parliament rather than by the executive. Basically, the Remainer claimants are arguing that the triggering of Art.50 is irrevocable, so effectively the government is using the royal prerogative to disenfranchise the UK people of their rights - something that constitutionally can only be done by Parliament. The government counsel has argued that Art.50 is reversible, which would undermine the claimants' argument. I've not seen anything about Donald Tusk saying it is reversable, but as a point of law the only court which would have jurisdiction to decide whether it is or not, is, ironically, the ECJ.