Two practical takeaways: (1) The smart thing to do is shake off your first impression and reevaluate after receiving new information. (2) Pay special attention to the first impression you give to stupid people.
Rest assured, there are plenty of blistering idiots on both sides of aisle, in equal proportions that match the idiocy of the general public (roughly 95/5). The amount of people who have the right combination of raw intellect, disregard for social convention, etc., to make independent and objectively defensible arguments that aren't simple regurgitations of the party line is probably 5%. This is true for basically every group and subculture (possibly except those that are so small they don't qualify as their own group/subculture yet).
Even among groups that attract primarily intelligent people in the absolute sense, like software or physics, most don't have the social courage to speak up when they disagree or to align themselves with someone who is clearly in the minority, nor do they have the interest to consider material in-depth and fully understand it in a way that allows them to internalize its principles and detect fakes or imposters. For the most part, they'll happily follow the crowd, no matter how silly the basis.
As a measure of cognitive ability, we used a 10-item vocabulary subtest from the WAIS. In this subtest, participants are presented with a target word and are asked to select the word from a list of five words that comes closest to the meaning of the target word (Cronbach α = 0.67, M = 6.96, SD = 1.90). The participant's score on this test was used as measure of his/her cognitive ability. Although the use of a subscale to measure cognitive ability may be less informative than full-fledged intelligence tests (see De keersmaecker et al., 2017), such tests tapping into a specific aspect of intelligence can be a valid alternative when administration of broad IQ tests are not feasible.
Although I don't think the study mentions "stupid people." And strictly speaking, you're probably not going to be able to administer even the 10-item vocabulary test before deciding how to make a first impression on someone.