Interrupt means it ends the channel/charge. Not exactly cancels it, but ends it.
Such abilities either have a persistent effect lasting as long as the channel (Fid's Drain), a channel that allows multiple casts (Xerath's and Jhin's ults), or a charged effect that changes its effect based on charge time-- usually its range (Vi's Q, Poppy's R, Pyke's Q).
But the charged abilities have a special aspect to them that channeled abilities don't. If held as long as possible, they do not fire. Every charged ability with a changing range (including Vi's Q) can be held until it ends without firing.
This makes Galio's W a special case. It's the only charged ability without that option. Holding it for the entire duration will still make the taunt activate.
So when charges are forcibly ended by a silence or other such interrupt, they're treated as if they had been held the entire duration. For most, that means they don't fire. But for Galio in particular it's just forced to fire-- which is still the same thing that happens if you hold it the entire duration, just like the others.
I hope that cleared things up.