People are taking this to lightly.

First: In Skyrim, you didn't time travel. You jsut looked back into the past and it took the strongest artifact in Tamrial just to do that much.

Second: The masks may or may not be time travel.

Third: If they have the ability to time travel, why do they want a weak bomb when they can rip the very fabric of time?

Im well aware. It was a form of Sword singing used by the Hiradirge.

I was assuming that the Breton's the OP mentioned used a form of magic which represented a nuke blast.

May only be able to do it at a certain time. My have a device that is good for one time use and can't transport an army or anything. They may have an elder scroll and simply are only risking using it because they feel the time is urgent enough. THere are so many possibilities as to why even within the confines of the Elder Scrolls lore as to what and why.

Didn't they send the main bad dragon forward in time? Is that not time travel?

Yes they did.

Alduin was never meant to be sent forwards in time, the nords tried to banish him from mundus but he accidentally got sent forwards in time which might have something to do with alduin being an aspect of god of time

And it took an Elder Scroll, read at a Tower, against a being made of Time itself.