Silence doesn't outright stop spellcasting. There are spells that have only simatic component. Sorcerers can go without both somatic and verbal components. And then of course there are abilities, that can repeat spell effects - but not spells, as such not hindered by sillence.
One of most obvious examples would be githyanki's mage hand. Even though it's repeat spell (mostly for simplicity, and probably devs laziness to write something new), it's nevertheless a psionic ability. Psionics don't have components that could restrict them (at least as obvious as spells can).
Did you read what he wrote at all? They are not talking about psionics or stuff that doesn't need chanting/sound to be cast or work.
Luring Song: The harpy sings a magical melody. Every Humanoid and giant within 300 ft. of the harpy that CAN HEAR the SONG must succeed on a DC 11 Wisdom saving throw or be Charmed until the song ends. The harpy must take a Bonus Action on its subsequent turns to continue singing. It can stop singing at any time. The song ends if the harpy is Incapacitated.
Silence
2nd-level illusion (ritual)
Casting Time: 1 action
Range: 120 feet
Components: V, S
Duration: Concentration, up to 10 minutes
For the duration, NO SOUND can be created within or pass through a 20-foot-radius sphere centered on a point you choose within range. Any creature or object entirely inside the sphere is immune to thunder damage, and CREATURES are DEAFENED while entirely inside it. Casting a spell that includes a VERBAL component is impossible there.
It would make sense for silenced enemies not be able to speak or shout to other enemies/characters either.