One thing to notice is these journalists / article authors have no clue about business whatsoever.
I remember James Rodriguez "earned back" his fee for Real Madrid after 2 weeks of shirt sales or some crap they posted back in 2014.
They concluded that 1 million shirt sales in the first two weeks multiplied by the average price of say €80 meant that they earned €80 million.
Revenue =/= profit
Once you deduct VAT, shipping, material cost, labour cost, (other production costs), overhead costs, distributor fees, etc you're left with a fraction of what the shirt is sold for.
I'd guess you United would get around 10 euro per shirt sold at most. But as others have mentioned, a Zlatan shirt sold might come at the expense of a Rooney shirt sold...