They're an assesment of many factors all encompassin Messis sporting contribution to FC Barcelona. No one are saying he's out on the streets of Barcelona selling cake and haircuts.
But here someone already did the legwork, sources included.
To be blunt, that work is simplistic garbage. Saying sources are included is generous, given that it amounts to them stating that their sources are Navigate Data Analysis and Forbes, without providing any links to actually substantiate their claims and the figures used.
They present 2 million Messi shirts sold as Messi generating 2 million shirt sales, which obviously isn't true. A number of those sales would have been made regardless, just with other players on the back. How many? We don't know, which is why it's impossible to say how much of the revenue is solely attributable to Messi.
And even with them doing their misguided and generous math, they end up with him generating €145m in revenue, over the €138m he's paid, stating this as a massive difference (while failing to factor in taxes.)