I agree with you Oldtimer or at least I wouldn't take the risk of believing the lower rate only applies to amounts in excess of €15,000. FA uses the words "entire" and "whole" interchangeably which to me seem designed to confuse rather than clarify. The lengthy debate about this in AAM seems to suggest the same.
I still think the wording is ambiguous. In contrast there is complete clarity on Rabo's site:First Active
For deposits of €15,001 and above, the gross rate of 4.33% AER will be applied to the whole balance.
First Active could just as easily have clarified their rates.For Example: If you deposit €15,000 with RaboDirect, you will receive 5.0%* interest on the first €10,000 and 3.75%** interest on the remaining €5,000