While supporting IE6 is onerous extra work, it's still work that can be done and has little impact on the final product.
Take the other position, that 'Not supporting IE6 will mean I'll charge you 70% of my original quote, only x% of people use it in your target market' or better add it as an extra in the quote in the first place as the article suggests.
Then it's up to them, it's a risk assessment. But be up front in your quotes with it, not an after the quote is won 'I don't know how to do it!'.
The article's incredibly OTT on how much adding IE6 support costs, but I certainly would never go down the route of lying to a customer about it. If he charges double it's probably because he hates doing it and fair game to him.