Okay, so video game insights is likely the better source for sales data on Steam then Steamspy then.
Steam spy has the number of owners between 20 and 50 million.
VGI has it at around 6 million.
No because I was informed the Belgian Embassy got their numbers from Video Game Insights, which at the time had the exact same numbers the Embassy gave to China. An Embassy in China is Not going to have access to Larian sales data, Larian is a private company, and whatever data they share with the Belgian government would not be something they could legally share with China, that would be huge privacy breach given to a foreign nation.
So no the VU/Embassy's numbers are not more reliable then Steam Spy why uses specific Valve API to get their numbers.