No, Megan McArdle is absolutely correct, as far as she goes; you can never pay your employees enough (per se) to make a profit purely on what they purchase. She just isn't going far enough, and anyone who's trying to implement her ideas needs to realize is that it isn't your goal to get your employees to purchase your goods and services; it is to make your employees value their jobs to the extent they don't leave you for greener pastures, and to reacquire the monies which they would have been spending anyhow. McArdle couldn't have chosen a worse example; cars are a luxury item. Buying a car is a big purchase, and many people, even today, just can't afford it, or it makes better economic sense to ride public transport. Food, clothing, these are not luxury items, which means that Joe Schmoe is going to buy it - somewhere - guaranteed. Those are the things that are not the 'extra' amount.
Joe's food budget is, let's say, 10% of what he makes; $X/10. You want to sell your food at a profit, sure. You want to sell your food to anyone, whether that's Joe or Myrtle or Henri VIII. If you're selling your food for 11% of what you pay Joe, Joe has to take that 10% somewhere else, and instead of you getting that 10% back - because Joe is just another frelling customer for a staple 'must-buy' item, basic food - someone else gets that 10%, and you lose a customer - and Joe, your employee, is disgruntled, because for the amount you're paying him, he can't even shop at your 'bargain basement store'. It'd be different if he were working for a high-end clothing store, or a car place, or whatever - but even then, it is wise to pay your employees sufficiently so that they find at least a minimal segment of your products are within their budget for that item. It's part of advertising.
Yes, you make your profit by selling to people other than your employees; that's absolutely true. That your employees shop where they work doesn't mean that you're losing your profit, it means that you're losing less of your labor expense. You're always going to be paying Joe $X per hour. Joe has to be making enough to buy food for himself and his family - if he doesn't make it with you, he has to go somewhere else and make it with them. If you make it worth his while to buy his food from you - because he's going to be getting food from somewhere no matter what - then you will be getting back the monies you paid him. And he'll be happy he can buy it from you, and he'll go home happy. Or be able, after four years of saving, to purchase that Honda, or wristwatch, or whatever it is you're selling. It's part of advertising; it's part of employee satisfaction. Customers pay attention when an employee says, "Oh, yeah, I got one of these last week/month/year. These things are great!!" You want 'word of mouth' to start with your employees.
Are your employees going to be your entire consumer base? Hell, no. But that's what McArdle's proposition subtly points out. She proposes that you lose money selling to your employee, when she's failing to account for the fact that for every one widget you sell to that employee, that employee (happy that she bought a widget, loves her widget, is enthusiastic about Wyrm-brand Widgets, They're The Best!!TM) is going to have sold more widgets. Because she's happy about them, customers who are interested in a widget but just want information right now (because Micronaut-brand Widgets are a buck less, just down the street) MIGHT buy today because Your Employee is so gosh-darned happy with her Wyrm-brand Widget (They're The Best!!TM) has convinced them with her enthusiasm and personal experience and satisfaction that they should not purchase a Micronaut-brand Widget, but a Wyrm-brand Widget, and find out for themselves why They're The Best!!TM
In other words, while McArdle's mathematics and conclusions are clear on the surface, she fails at Business Employee Sense 101 - and fails miserably.