I know "Diploma" is something less to the Bachelor in English-speaking countries. But i think it was wrongly translated to English on the royal website, because the degree given in the other languages (German, Dutch) is a lot higher and equal to a Master.
If it is good translated to German Mathilde has "ein Psychologiediplom cum laude" (see heer Monarchie - Prinzessin Mathilde
). In Germany this would mean the highest examination in a subject below the Doctor. In Germany a "Diplom" is totally equal to a Master.
Here Mathilde d'Udekem d'Acoz - Wikipedia
is said she has an Licentiaat, which is equal to an Master (see Licentiaat - Wikipedia