This issue is fixed, and it required [a replacement of the crank][1] in addition to the front pulley. The belt wasn't the issue. My initial question was the wrong one.

I've accepted [Nathan Knutson][2]'s answer above because it describes the troubleshooting approach that led me to the solution.

But for completeness for anyone maintaining their Spot Acme, Mitchell, a tech at Spot Bikes, answered me via email saying that **the belt port should be tightened to 9-10 Nm.**

Here's his full email response, which also describes a technique for troubleshooting the issue and general specs for belt tension on the Acme. I've emphasized the key ideas:

> I'm sorry to hear that you are running into some issues with the belt
> drive setup on your Acme! I do hear what you're talking about in the
> video, but the interesting thing to me is that **it appears to happen at
> the same point in the crank rotation on the front ring, which wouldn't
> correlate to the belt** since the drive ratio isn't 1:1. If you have a
> white or silver marker, I would suggest putting a little dot/line on
> the belt and the ring where it starts making noise, and then rotate
> the cranks and see which one it lands on consistently when it makes
> the sound. 
> 
> Another thing to consider - remove the rear wheel and pop the belt off
> of the front ring. Once it is off, rotate the cranks and keep an eye
> on the front ring to see if it hops or wobbles at all. I would assume
> that you've inspected the belt for damage already and that when the
> bike fell, there were no signs of it landing on the belt correct?
> 
> **The torque spec on the frame removal piece to replace the belt is
> 9-10Nm.** Theoretically, replacing the belt in a like-with-like scenario
> wouldn't automatically require you to re-tension it, but I would
> absolutely check the tension to ensure it is in the proper range
> before riding to avoid any potential issues. **We normally use the Gates
> Carbon Drive app to measure tension, which should be 35-50Hz for the
> Acme.** 
> 
> Thanks,
> 
> — Mitchell

  [1]: https://vimeo.com/802183140
  [2]: https://bicycles.stackexchange.com/users/25959/nathan-knutson