Where was the main gun they were talking about?
In the same place it always was? Firstly, remember we didn't see the whole battle, just the initial volley and later when the Alliance turned up. There's a gap there that could have been mere minutes or hours that we never saw. On the other hand the main gun could have been knocked offline with the first shot.
Secondly, the Destiny Ascension wasn't trying to engage the fleet, they were evacuating the council while the turians engaged Sovereign. You can very clearly see the ship flying *away* from the battle before the reaper closes in, probably making for one of the other mass relays.
Thirdly, a main gun like they is probably only effective against large and preferably stationary targets, like cities and space stations. It wouldn't do a lot against a swarm of smaller ships. At best it would only take out a handful, if they don't just simply swerve to avoid the projectile.
Fourthly, the ship does open fire if you look carefully (around 0:36)
. It may be the main gun, it may not, there's no way to tell, but we did see it get a few shots off.