Is Sedna's "silence" skill an interrupt?

It interrupts.

http://forums.demigodthegame.com/404849

It is not an interrupt, an interrupt causes cooldown if it blocks a skill/item in progress.

There are 4 kinds of blocks

Interrupt
Stun
Freeze
Silence

The difference between Interrupt and stun is simply the text you get. eg: Although every interrupt is set to a 0.1 duration, if you set it to 3, it would work like a 3 seconds stun with the exception of the text.

Freeze will stop skill animations immediately, make a "freeze" sound effect and turn the character blue. But it does not cause skill coolodown.

Silence does not block a skill use animation, but it does stop the affect from happening.

eg: The difference between a freeze and a silence on something like hammer slam is that the freeze will stop the animation, where as silence will let it complete, but cause no effect.

All of them will block skill / item usage during the affect duration.

Now you may ask yourself does the difference between freeze and silence really make that much of a difference?

It does if you are modding and want to make an "interrupt" that does not cause cooldown. eg: a 0.1 seconds freeze vs a 0.1 second silence. The freeze will block the skill/item in progress because it stops the animation where as silence does not.

eg: Going back to the hammer slam... a 0.1 freeze will block it, where as a 0.1 silence will have no affect, unless the slam happens to land and cause damage at precisely the time the 0.1 silence goes into effect. (at which point it will block the slam affects)

LO is 100% correct from a purely technical point of view, of course. However, since the OP is clearly interested in practical applications of Silence (does it prevent locking? can it stop ports? etc.), we can agree on using a somewhat simplified terminology here

Silence and Frost Nova simply "delay" the skill for the duration of the debuff. Even if you get your target mid-cast they will not suffer a cooldown.