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A scholar wrote in a very learned British magazine this sentence:

"[The investigator] worked out from the actual area of Tahrir Square and the streets leading to it that on the most generous estimate the demonstration CAN'T [my emphasis] have exceeded 265,000 people."

Why did he use "can't" instead of "couldn't"?

Thank you


Source: [Professor] Hugh Roberts, LONDON REVIEW OF BOOKS (print edition), September 12, 2013, page 5

I have no idea. "Couldn't" is strictly required here, although tense concord rules are occasionally ignored, even by well-educated natives.