You're right that Shin Jinseo is a generational talent, and more dominant than anyone since Lee Changho (peaked in the 90s and was strong into the early-mid 2000s).
However, you can't compare goratings over time, the top ranks are not nearly stable enough. https://www.goratings.org/en/history/ (I think it's believable Shin Jinseo is better than Lee Changho, but not that there has been steady progress since the days of Lee Changho, so that there are now 20 players stronger than him).
The problem is ambient go knowledge. A top 100 player would easily beat time traveling Lee Changho in his first few matchups. Of course give peak Lee Changho a fortnight to prep with Katago and … well that would be something!
You can't compare Elo ratings over long stretches of time, period.
Ratings drift over time, based on the total population of people competing. I think the most accurate way to view Elo ratings is as a measure of skill vs. the average rated player.
If you want to compare Magnus Carlsen's peak rating of 2882 in the year 2014 to Garry Kasparov's peak rating of 2851 in the year 1999, you have to know how strong the total pool of players (including all the amateurs who compete at lower levels) was in the year 2014 vs. 1999.
The only way to actually anchor the Elo system over long periods of time would be to have rated humans occasionally play against a set of unchanging computer players, which could then serve as static rating calibrators. You could use those games to then calibrate Elo ratings from different time periods to a common scale, by asserting that the computer ratings don't change.
Even that would be slightly malinformative because of opening theory. If you warped a very strong player from the past to the present, he'd do very poorly at first simply because of advances in opening theory. But give him a bit to catchup and he'd likely have his rating zoom on up. So modern players would do better against the static computer because of the same advantage, but that doesn't mean they're necessarily stronger in the sense that we hope to measure. The question people always want to know are things like how would a Morphy, Capablanca, or Alekhine do in modern times with access to modern theory and the like - not how well would they do against Carlsen if they went in with nothing but the knowledge of their era.
Could you just have superhuman Go AI just how good humans are somewhat more objectively?
Not without flaws of course, but probably interesting