Good question Michelda. It is not because of the relationship between TAS and IAS. It is probably just a mistake.
Whether you talk in TAS or IAS the Vx and Vy converge on the same value at the absolute ceiling. It is impossible for the Vx to increase above Vy. Your question implies you know this already, and your suspicion is correct. If there is a rational explanation for this not being a mistake I would be very pleased to read it. Definitely, nobody has managed one so far.