My opinion is that it's not too far off the figure. Considering he is a global sensation and so are Barcelona.
He has 245,000,000 followers on Instagram, so 2,000,000 shirt sales is a fraction of that amount.
So lets say hypothetically speaking a shirt costs £50, I have no idea how much they cost these days. £50 x 2,000,000 = £100,000,000.00.
Now when you think Argentina has a population alone of 44,940,000 people again 2,000,000 is a fraction of that. How many kids who idolise Messi would buy a Barcelona Messi shirt?
I could go on and on to be fair 1,920,000 people live in Barcelona.
How many kids do you see just out and about in the UK alone with Messi shirts on?
Barcelona tourism, I haven't even touched upon that. Figures suggest 32,000,000 people visit Barcelona each year. Now I wouldn't like to put a percentage on it but even if you put say 25% (8,000,000) of people that visit each year come visit the stadium, go for a tour, buy some merchandise, go to the match. Out of the 25% (8,000,000 people) say another 20% of that 25% (1,600,000 people) buy Messi shirts that's a further £50 x 1,600,000 = £80,000,000.00.
A lot, a lot, a lot of money generated, and that tourism has just been wiped out this last 18 months.