There is one important point that we have not discussed in this topic.
That is how big difference is between 4d and 5d, and 5d and 6d.
From my experience, I started playing at age of 14, and after playing for 4 months some 5-6 quick games a day with my brother, we became some 5k. (that is 1k / week). Only with playing.
After that, since I did not have proper teacher, I had to work more on theory and read books, and it took me some 1,5 years to reach 1 dan (total of 2 years).
To reach following categories, it took me a year for each dan, and I reached 4d at some 19 (5 years of studying). It could be faster if I had coach, more books, etc.
Next, there was great difference for me between 4d and 5d. That was difference to reach top players in country.
After studying with them, it was necessary to read even more books. I had to move to stronger, Japanese literature, and since I don't know to read it, it was difficult to understand. Finally I realized simple recipe - player who plays last in diagram is not worse in situation, it is draw or he is better.
At peak, being strong 5d (2523) which some might say is pretty close to 6d, I realized just how far it was:
by looking at players with greater rating, I spotted that all of them had either spent some time studying go on Far east, or were from countries with several 6d+. (same as today, btw)
http://www.europeangodatabase.eu/EGD/Find_Player.php (sort by last column)
That meant I would have to spent lot of time studying, playing on strong tournaments, travelling to Japan - basically devoting my life to that for more than a year, which I could not do since I started to work, and had no support from Go Federation. And for that level, you have to be strong at all segments of game, you can not just count on your talent at one segment, it is necessary to study everything. And all that work for some mere 50 rating points.
That is why it is no surprise that so far there was only around 100 players from Europe who reached 6+d level. Of them, majority spent months in Far east studying go, or were trained in strong local centers.Therefore, when we have a case that someone jumped from 5k to 1k quickly, sure, it is easily possible.
From 1d to 3d? Yes, even without too much reading.
And that someone who was 4d plays like 6+d?
Sorry, sorry, sorry. It is just insult to European best players who kicked their ass of studying to become that strong.
Please bear that in mind when we discuss this case. Lot of work has to be put into it, and it can be easily shown.