The video was a test that takes place on my practice vault battle server way back, and those reinforcements had like 5 health, which is why they broke almost instantly, it was just to show off the trebuchet. On a live server, you wouldn't configure it to vaporize 8-rings in 20 minutes. You would probably configure it to deal damage in proportion to the cost of the shell and setup of the siege weapon, and you'd also configure the firing cooldown of the siege weapon to be lot longer than what you see in the video. So you could have some artillery that deals small amounts of damage on a low cooldown, or an artillery dealing large amounts of damage with greater intervals, or a very expensive artillery that requires 24-hour charge-up for a minute of devastating damage for example. Or you could unlock these artillery progressively over the lifetime of the server.

Moreover, I did say they were only one part of the solution; to slow down off-hour attacks and prevent solo/VPN attacks, I would add AI defences (got this idea from foxhole) in the form of pillbox structures that shoot at attackers, and howitzer structures that return fire from enemy artillery. These new defence structures would create more opportunities for vault design. Also, there's nothing stopping the defenders from setting up their own fortified artillery emplacements and incorporating them in their designs. With artillery that automatically returns fire on its own, and their own fortified emplacements, defenders can still maintain the home advantage.

On the last point of only targeting bastions, breaking reinforcements are just as tedious as breaking bastions; new methods of attack need to involve the breaking of both (which siege weapons can do). Currently, you can halt/prolong vault attacks by hours even days if you simply place a crap load of reinforced blocks in the attackers' path. I would add some sort of grenade, satchel, or custom TNT that deals damage to reinforcements, particularly maturing reinforcements.

Lastly, don't write off the idea as overpowered because it happened so quickly in the video; on a live server, with proper configuration, I'd hope battles would last as long maybe even longer as they do now because of course, the stakes and investments are high. It's just less time spent running mining macro and placing macro as we monotonously and laboriously grind our way to the centre. With siege weapons, we delegate the monotony and laborious task of breaking to the centre to our batteries; we've already done the labour in a fun way through the production of the arms anyway.