>"Something like this is a pretty standard and unsurprising result."
No there's actually nothing standard unsurprising about it. GDP before the industrial revolution meant something entirely different than after it.
That infographic was largely debunked when it came out. Most notably:
"Before the Industrial Revolution, there wasn't really any such thing as lasting income growth from productivity. In the thousands of years before the Industrial Revolution, civilization was stuck in the Malthusian Trap. If lots of people died, incomes tended to go up, as fewer workers benefited from a stable supply of crops. If lots of people were born, however, incomes would fall, which often led to more deaths. That explains the "trap," and it also explains why populations so closely approximated GDP around the world.
So, one way to read the graph, very broadly speaking, is that everything to the left of 1800 is an approximation of population distribution around the world and everything to the right of 1800 is a demonstration of productivity divergences around the world" [1]
>"a) This continued well into the 19th century, and"
No India's economy in 19th century is very much the British economy the is was the time of the Raj.
>"b) India was bigger than China in medieval times."
This is also untrue. The Medieval era some 900 years(end of the beginning of the 5th until the 14th century.) India was not bigger than China during this millennium. India in the Medieval Era was ruled by many regional dynasties[2], while China was unified by the 3rd century BC [3].
[1] https://www.theatlantic.com/business/archive/2012/06/the-eco...
[2] https://en.wikipedia.org/wiki/Medieval_India
[3] https://en.wikipedia.org/wiki/Qin%27s_wars_of_unification