Having some experience with studies of polarization, maybe I can help clear up where this paper comes from.

Basically, there are at least four distinct types of polarization that are commonly studied:

* affective polarization (used in this study) measures hatred towards the outgroup. It is commonly measured with the feeling thermometer, alternatively with the gallup question "would you agree if your kid marries someone from X". This polarization has been increasing over time, and recently outparty antagonism in the US surpassed (for the first time) racial antagonism.

* ideological/partisan polarization measures diverging support for the two parties, i.e. voters drifting towards the edges of the political spectrum. Note that this does not require that parties also become more extreme.

* elite polarization occurs in politicians and can be identified from speeches etc., but also from co-sponsoring of bills

* perceived polarization is the perception of how polarized society is

Now, the interesting thing is that many of these phenomena have (at least) two distinct causes: Either sorting, i.e. people realizing which party they really want to vote for. This occurs when parties become more extreme and it therefore becomes easier to identify which party is ideologically closer. Alternatively, people themselves may (also) shift ideologically, creating a clearer gap between parties and their electorate.

Most research is done on the US due to its simple two-party system, but there is still some debate regarding the true scope and causes of polarization. For example, one paper could not find a clear causal relationship between internet penetration and different forms of polarization [1].

If you're interested, some good references are listed at the end.

[1] Boxell, L., Gentzkow, M., & Shapiro, J. (2017). Is the Internet Causing Political Polarization? Evidence from Demographics (No. w23258; p. w23258). National Bureau of Economic Research. https://doi.org/10.3386/w23258

Abrams, S. J., & Fiorina, M. P. (2012). “The Big Sort” That Wasn’t: A Skeptical Reexamination. PS: Political Science & Politics, 45(2), 203–210. https://doi.org/10.1017/S1049096512000017

Fiorina, M., Abrams, S., & Pope, J. (2004). Culture War?: The Myth of a Polarized America. Undefined. https://www.semanticscholar.org/paper/Culture-War%3A-The-Myt...

Fletcher, R., Cornia, A., & Nielsen, R. K. (2020). How Polarized Are Online and Offline News Audiences? A Comparative Analysis of Twelve Countries. The International Journal of Press/Politics, 25(2), 169–195. https://doi.org/10.1177/1940161219892768

Levy, R. (2021). Social Media, News Consumption, and Polarization: Evidence from a Field Experiment. American Economic Review, 111(3), 831–870. https://doi.org/10.1257/aer.20191777

Stroud, N. J. (2010). Polarization and Partisan Selective Exposure. Journal of Communication, 60(3), 556–576. https://doi.org/10.1111/j.1460-2466.2010.01497.x

Tucker, J., Guess, A., Barbera, P., Vaccari, C., Siegel, A., Sanovich, S., Stukal, D., & Nyhan, B. (2018). Social Media, Political Polarization, and Political Disinformation: A Review of the Scientific Literature. SSRN Electronic Journal. https://doi.org/10.2139/ssrn.3144139