[ferzul]

Fortunately it is not a claim in the text you quoted. If a change is necessary, it has already happened everywhere. So neither t-flapping nor th-fronting are necessary, but both are more likely than [t] > [k]. There is no specific reason that one of them advanced through one country and the other spread through another, even if there is a good reason the third change hasn't happened. Or equivalently with the abandonment of “reckon” vs the substitution of “autumn” for “fall”.