It doesn't say that in the piece you highlighted. It says that if window cleaners were forced to pay VAT, income tax and NI, that the economic contract may not have happened at all.
In the EXAMPLE it provided, it's pointing out that an increase in tax inspectors does not necessarily mean they will bring in extra cash to the government coffers, because the threat of the inspections will mean prices in window cleaning may have to rise, which means people will not purchase those services anyway; therefore no extra government cash.
I haven't read the whole article though; not closely; so maybe you're referring to another part of it?