Successful freelancers need to charge at least double the hourly rate that they'd be happy earning doing full time work. (In many fields, it's more like 4 or 5x).The implication Godin is making is that freelancers need to charge more so they aren't busy with work all the time, and can therefore afford to spend time getting better. Which is reasonable logic.
And they need to spend at least half their time getting better at their craft (and helping the market understand and appreciate what they do).
Your mileage may vary, but one sure route to becoming an unhappy freelancer is charging just enough and hoping that the low price will keep you busy all the time.
It makes sense as a shorthand that you can drop in a quick blog post, but it sidesteps at least some significant considerations for the freelancer her/himself. Health insurance, for example. And retirement savings. Basically, the costs of living that you often don't need to think about in the short term. You need money now to pay for a roof over your head, food, the tools you need to ply your trade, etc. Those types of thing are immediate and obvious. Unless you have immediate health issues, insurance might not be. Unless you're getting on in years, retirement might not be. But both are significant and worth considering.
The average savings rate in the U.S. is currently around 5½% -- not an all-time low, but it's definitely pretty low when you compare it to the past several decades...
Of course, as a freelancer, you don't have to retire at all. If you're really happy doing your thing, and people are still willing to pay you to do it, by all means, carry on! Joe Sinnott is 88 and only formally retired a few years ago, but continues to ink the Sunday installments of the Spider-Man newspaper strip and continues to do some commissions and odd freelance jobs. He recently put together a Kickstarter collecting some of his work from the 1950s. Larry Lieber is 83 and still draws the daily Spider-Man strip. Creators like Milt Caniff and Charles Schulz essentially worked right up until their deaths at 81 and 77 respectively. But that's all dependent on A) your health (particularly your eyesight and hand-eye coordination) remaining largely intact, and B) your work remaining in demand at some level. There's been more than a few creators whose work fell out of favor because they were out of step with contemporary society or they wound up repeating themselves frequently.
Bottom Line #1: You can't count on being able to work forever.
Bottom Line #2: You need to prepare now for your life as best as you can for a post-freelance era.