I was going to make a similar comment, after having drawn the association above.
What the study does show is that the Sturgis variant was epidemiologically detectable throughout Minnesota following the rally. Note that the total number of samples was drawn from identified interviewed patients only, though this appears to have covered all reported cases in MN during the period (August 1 -- August 31, 2021). My understanding is that the survey was as comprehensive as could be achieved, and not a limited sampling.
During the same period, the 7-day average new-daily-case rate in Minnesota was 699--795 (rising slightly over the course of the month, observed from Worldometers). The Sturgis-linked cases were 0.4% or less of total confirmed cases.
(I'm assuming the CDC tracing was comprehensive, the paper is less than clear on this, though that appears to be the case.)
We could make numerous arguments that the study was flawed, missed cases, was incomplete, etc. All of that would be an argumment from ignorance. And though I feel there's some case to be made for suggesting the CDC undercounted, that leaves us with a weak basis for any further conclusions. Sometimes, though, in epidemiology, that's the best that can be done, and the precautionary principle kicks in: what course of action would provide the maximum benefit and least harm.
Minnesota's Winter Wave really started spiking in October of 2020. It peaked on 20 November at 7,023 NDC (7-day average). Whether or not those cases are linked to Sturgis would require sequencing of a sample and drawing inferences.
That said ...
... other evidence comes from looking at the spread of Covid-19 hot-spots throughout the US from August -- January of 2020--2021. And that did show the radiating pattern I described. I'm not certain it's associated with Sturgis, but it very much walks like a duck.