South Korea is an oligarchic democracy, and, as the paper points out, has developed deep political divisions between the left- and the right- in its society.
The deposed South Korean President was a favourite of the Biden administration ( https://www.rand.org/pubs/commentary/2022/07/yoon-suk-yeol-i... ) because of his pro-western and pro-business values and his political willingness to militarily commit to western / US foreign policies against China and Russia. (He is the first South Korean President to attend a NATO summit, has been public about his desire that South Korea be a part of an "Asian NATO" - Quad + Jarokus + AUKUS - supported sanctions against Russia and has even supplied Ukraine with the artillery they badly needed).
Some of Biden's and Yoon's foreign policies seem to have backfired on South Korea as North Korea's near-total alienation, that previous US and South Korean administration had achieved, to economically and military weaken it, is now over. (Russia and North Korea have signed a new political and military alliance, committed to help each other if the other is attacked and, North Korean troops will not only get upgraded armaments from Russia but are also getting battle trained in real warfare conditions in the Russia-Ukraine war). Many South Koreans are not thrilled about this, because the most serious and imminent threat for South Korea has always been from the North Koreans. While many South Koreans understand that a military alliance with the US means that they cannot disregard US foreign policy concerns, many in the left- are concerned that there was still enough room to manoeuvre to not aggravate both China and Russia so much that they are now openly hostile against South Korea too.
I believe this was one of the underlying tension between the Yoon's administration and the opposition - instead of trying to create a democratic political consensus in South Korea about his policies, he suddenly decided to deal with valid criticism about his domestic and foreign policies by attacking the opposition as “pro-North anti-state forces”. (This was stupid as even though latent anti-war sentiment amongst the South Korean working class is high, the left-leaning opposition Democratic Party are also inclined to the idea of joining the US lead anti-China military alliance and a consensus on foreign policies atleast was very much possible.)
His political bungling and attempt to tarnish the opposition as North Korean stooge now means forming political consensus will be now more difficult and further polarise South Korean polity, thus allowing Russia, China and the US to interfere more in their political affairs.
I fear democracy in South Korea is still under threat, just it is in many other parts of the world facing similar political polarisation.