That's a great point, however, I don't think it challenges the findings, rather, your points go some way to explaining some of the drivers.
I should add the Elephant in the Room because nobody has yet - which is social media:
Twitter and Facebook are 'primary drivers' of this division, because of 1) the pervasiveness of content 2) the low quality, non-factual nature of the memes and 3) the natural polarisation of such memes.
More controversially, 'campus politicisation' is another primary factor. Students have been protesting for some time, but now the Faculty, Staff, Students, Student Unions and government grand associations act in concert to promote what they see are important issues, but which really represent a politicisation of the situation.
There are innumerable cases, but just one anecdote: The University of Ottawa recently banned the Yoga class because of concerns over 'cultural appropriation'. Never mind the fact that 'Hatha Yoga' in the North American terms was literally created by Hindu leaders as a form of their theology more amenable to Westerners in the first place. The replacement Yoga teacher was of 'Indian' origin, of course. Nobody complained, rather, someone on faculty out of either fear or oppression decided that it would be the case.
Finally, and maybe most controversially because its more subtle - is the politicisation of corporate culture and brands. It's one thing to have internal dialogue and programs, this is fine and good, but when they're hustling products based on highly visceral movements ... it's going to help stoke division.
It's not well understood by most people that products and services are sold mostly on the basis of emotional aspiration. Nike has great shoes, but that's not what drives sales. They didn't really make money until they had 'Michael Jordan' on their shoes, after which, they made bank. Starbucks sells a total commodity: coffee - what they really sell is a brand and an experience. Right down to the naming of the water 'Ethos' as in 'I'm ethical for drinking this water'.
But the recent foray of brands into taking sides on social issues, which many who are supportive of such issues might admire, should be viewed cynically: in the end, they're just trying to sell you some crap. If you 'feel strongly about this issue' - and they can map those emotions into sales by 'siding with you' - they're going to do that to make money. Again this is separate from things like material action such as ethical supply chains etc., but when Nike pays people in Vietnam 50 cents an hour who can't take a bathroom break to make 100 shoes a day, and then sells them to you for $200 'because they care about this issue' - we should be cynical. This action contributes to increasing polarisation.
In a way, some political issues are 'metastasising' into so many areas it's getting to be like the Twilight Zone.