I love that HN hyper-focused on your 600 points observation. However, 600 points means you went from 1500 (the start) to at least 2100, which is almost universally recognized as "pro" or at least semi-pro for chess.
So yes, logarithmic curve and dimensionless pedantry and all that, I'll grant you; but that's missing the trees for the squirrels in the forest, since 600 points means "I went from noob to pro." No one expects Magnus to gain 600 points, since that's quite impossible.
I guess you could argue that it's possible for someone to start at 1500, then really suck at chess and drop to 900, and then merely become average again (1500), and claim a 600 point improvement. As with the other observation, I agree, that would be impressively misleading. I'm not sure that's the claim, though.
EDIT: I retract every claim. I am in fact an idiot, since the article does say they went from 1200 to 1800, not 1500 to 2100.
This is what I get for writing something dumb. At least I admit it right away though. Sorry.
At least it's proof that the pedantic-ness wasn't so pedantic.
I also take solace in the fact that the actual title of the article was stripped: the title says 1200-1800, not merely 600 points, which in this case is crucial info. But! It was remarkably stupid not to actually click on the link before writing, and this is rather public proof that sometimes I don't. Perhaps that's a strong signal that in the future, I need to be. :)