The board you linked isn't a galvanic isolator, it's just a capacitor bank covering a bunch of different frequency ranges. The ADUM isolator I linked as well a

The board you linked isn't a galvanic isolator, it's just a capacitor bank covering a bunch of different frequency ranges. The ADUM isolator I linked as well as flyback converters use transformers to provide a high-voltage isolation barrier between the source and device being supplied. Transformers are generally larger, more expensive, and less efficient than non-isolated DC circuits. The POE supply itself is isolated (with a transformer) and doesn't need to be noisy necessarily - but if you want even more quiet, another level of isolation can't hurt.