Thyroid function may be affected by carb intake, but really it's caloric intake that makes the biggest difference. Low carb diets tend to blunt hunger and ake it harder for a lot of people to get adequate caloric intake on a daily basis. Time Ferriss was probably basing the "massive caloric load" recommendation on the need to make up for the displaced calories that result from a lower carb diet typically.
And he was assuming, most likely, that on his plan you'd be working out so that you would need extra calories to begin with. Besides that, the cheat day gives you a chance to relax and not worry about what you eat for a day. While I don't think you need to make it a weekly thing necessarily, having a cheat day every once in a while, as he argued, also increased compliance with the dietary plan the rest of the time, actually increasing the positive results of those who implimented the plan. You no longer have to feel guilty if you go off for a day or two, it's ok, it might even be good for you and you can hop back on the bandwagon once again. He recognizes the realities of modern life.
Again, consuming anything not primal is not the optimal choice, even for a refeed. However, given the realities of modern life (see above), this is not always an option. Plus (as above), allowing yourself a few cheats every now and then without killing yourself over it can increase compliance with the diet and/or lifestyle you are following.
Refeeding serves the purpose of keeping your body "in check." If you only opperate off of one fule all the time, you cannot use the other one very effectively. Maintaining a body that is capable of utlizing all sources of fule is ideal should you ever be placed in an environment where you might not have access to a food source that you consider to be "optimal." In addition, there are certain benefits to consuming small target doses of carbs from time to time such as benefits to thyroid function (though these have been debated), keeping your metabolism active (so as not to stagnate it), and others.
Somewhere a while back I read the theory (no idea if it's true), that consumption of carbs actually increased muscle absorption of protein. Essentially, those who ate carbs AND protein absorbed more of it into their muscles mean both that they built muscle more efficiently and that they didn't need as much overall protein since they absorbed it better. This was a hottly debated topic however and I believe no definitive word has been laid on the matter as yet.