A major confounding factor is everything else in the air. Humans produce lots of different gases, and CO2 is usually a proxy for the overall concentration of our effluent gases. But in a submarine, or in some buildings, there are gas filters (usually carbon, possibly with various modifications) that can remove or destroy some of these gases but have no effect on CO2. So the air in a submarine at 15000ppm CO2 could be very different from the air in a an unventilated room that reaches 15000ppm CO2.

I don't think you can cleanly compare this: In the study, they added CO2 to the room, while keeping O2 at normoxic levels throughout the experiment. In your meeting room, O2 levels will be dropping in lock-step with the CO2-levels rising. It may be the lack of oxygen that leads to drowsiness, not the additional CO2. But it's the CO2 levels that you can measure as a good proxy of overall air quality.

If that study was of submariners, is it possible long-term high-level exposure causes the body to adapt?

I am suspicious of 0.1% having a significant effect though, given oxygen is around 20% and we naturally exhale a couple of percent CO2.

Though that study included a 45 minute acclimation period. Appropriate for submarines, but I wonder what the results would be in the first 1 / 5 / 10 minutes.

... which is entirely unsurprising given that exhaled air is about 50.000 ppm CO2 and can vary by several 10.000s depending on depth and rate of breathing. I actually consider the recent wave of findings that CO2 levels as low as 500-1000 ppm measurably affect cognitive performance and well-being to be a great example of how you can prove literally anything with statistics and a sufficiently small sample size.

could be a selection effect at work

One key difference is that submariners are rigorously trained to operate effectively in less-than-ideal environmental conditions, whereas Bob from accounting probably is not.

Submarineers also are hand picked due to their great lung capacity...