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).
