If I have the numbers correctly in my head, Ferdinand is 13 years older than Myne (not counting the 2 year coma or age-shift for Rozemyne’s fake noble birth), and Benno is another 8 or so years older than Ferdinand. While 13 is a lot, 21 is still quite a bit more… At least Ferdinand isn’t old enough to comfortably be is future wife’s father, which Benno is easily.
Didn’t he say that already once, when she charged into Ahrensbach to save Ferdinand, while Sylvester was stuck in his own duchy, due to knowing of the imminent invastion? Or is my memory playing tricks on me?
Either way, here I think it shows that Sylvester noticed (and proofes) that for the first time since Sylvester met him, Ferdinand is doing things for his own wellbeing and not sacrificing his all for others.