I would like to point out that an Asteroid of similar mass is going to be significantly smaller in actual dimensional volume. One thing to remember about your ships: They're hollow! That outer hull is hiding a latticework or honeycomb of open air bubbles. A lot of the armor is on the outside, like the shell of an egg. To armor up the interior walls is probably going above and beyond, so a budget conscious ship design probably isn't going to have solid bulk-heads throughout that can withstand the same blast as your primary armor on the outer hull.
While those Star Destoyers in ESB may have been womp-ratting a great deal of those asteroids, their shields were still weakening from impacts, to the point that the ships were taking mission-kill damage.
What Triptych is describing is essentially the most basic projectile known to man. An arrow or spear was, at its earliest inception, a Rock on a Stick! Slung from a spring , arrow or arm.
A bullet, by any other definition, is a solid lump of ore. How it's projected to speed is variable. Even lasers or plasma beams are essentially the same thing, the volume and size of the bullet being the only real defining factor.
An asteroid arrow is simply a really big bullet. And, if you pick the dense, metal ones, like a shepherd boy picking out stones from a spring for his sling, you have something that is pretty compact. You scrunch your cruiser down so that you don't have all those bubbles taking up space, and you have a significantly smaller target to shoot at. And, if done right, there's no need to defend it. Heck, think of the mass drivers from Babylon 5. You could probably carry the things like ammo.
So, conceptually, there's no reason not to have made and used Light Speed Asteroid Bullets.
It's possible that it was a tactic used and abandoned millenia ago in the Star Wars Galaxy's timeline. Maybe it's historical lore, and the inspiration to execute the maneuver came from memory of a story somewhere. Hopefully they come up with some explanation as to why it's not more common a tactic in large fleet engagements.
But, questioning the common use of the tactic shouldn't be dismissed out-of-hand, either.
Until they give us an official answer, however, we can only speculate about the giant elephant asteroid in the room.
#P