Yes. It's simply not mathematical.

Suppose, x^(x^(x...) = y

At first the author says --> y = 2 and when he says "now consider the equation..." he equivalently says --> 4 = y.

Then obviously we can say 4 = 2. Means, you are assuming 4=2 and then concluding 4=2. Please point out if I am wrong. :)