I can think of a couple possible explanations for the 60% limitation.
First, as you know, Tom advises mainly professional pizza operators who read the PMQ magazine (and/or the PMQ Think Tank). And, in a professional setting, it is not at all uncommon to see hydration values below 60%. The lower hydration values makes it easier for workers to work with the dough.
Second, if the hydration values get too high, you need the correct amount of oven heat to get the dough to rise in the oven and get good oven spring. As scott123 has noted, Tom doesn't discuss oven temperatures. He also doesn't mention the the style of pizzas he has in mind. For example, many pizza operators who specialize in the NY style pizza use temperatures that can go below 500 degrees, along with fairly long bake times. That is unlikely to be high enough for high hydration doughs.