You don't have to pay VAT on things you fix for yourself, because you don't pay yourself at all. This is in fact a kind of (legal) tax avoidance, but not (illegal) tax evasion. Given the cost of housing, being able to build your own house or even just doing small fixes here and there, leads to a big increase in perceived income. The tradies I know can afford whatever kind of car they want, whatever kind of holiday experience, and they live in a nice home. Mind you, they typically work 50h+ a week so there's that.

Of course, the parent may also have been referring to getting clients to pay in cash and not putting anything on the books, at the expense of getting barely any pension in the end, but that's not how I read it. This is getting somewhat less common because people are more likely than 20 years ago to get a loan from a bank to pay for renovation work, and the bank will want to see invoices.