Part of the goals of proselint is to minimize the number of false positives that traditionally clutter the results of style checkers, resulting in users ignoring the changes when they see them. We want to be reasonably certain before raising an alarm. You can read more about the precise metric[^fn1] we use here: http://proselint.com/lintscore/.
And yes, `python3` for the win. :)
[^fn1]: If you wanted to be truly precise, it's a parametric family of metrics.
I ran it on a couple 800 word emails and it didn't catch anything except me using 2 spaces instead of 1 in one place. I also ran it on my city's sidewalk maintenance ordinance, and it didn't report anything.