To me it's clear Clifton is a) not a flat earther, and b) is obviously doing a thought experiment in order to demonstrate how to be more flexible in your thinking. I'm sure he is not insisting that the planet is accelerating outward in all directions at g, because skydivers accelerate just as quickly toward the earth over Australia as they do over Canada. The test for these things (whether they are some equivalency for mathematical simplification, or not) is usually the conservation of energy. That the planet is expanding at an increasing rate such that the surface all over the globe exerts an acceleration of 9.8 m/(sec^2), or it is careening of in some convenient observer preferred direction at the same increasing rate does not conserve energy.

What he is saying, if you read the whole text available, is that the earth is preventing you from following your free path in the curved space around the mass of the planet, and he's just giving you another way of thinking about it - which is just a restatements of Einstein's equivalency. And, there are limitations of to equivalency - the aforementioned conservation of energy is the prime, but not the only way of telling the difference between gravity and acceleration in real situations (as opposed to interesting, but academic thought experiments).