That's not a good summary - later in the answer author shows a way to go beyond FSMs, up to deterministic pushdown automata. They sidestep the issue of addressa

That's not a good summary - later in the answer author shows a way to go beyond FSMs, up to deterministic pushdown automata. They sidestep the issue of addressable memory by using the call stack and unaddressable `register` variables. C implementations are allowed to have no limit on recursion depth and have unlimited `register` variables, allowing us to pass data between caller and callee without using addressable memory, which gives us the power of DPA, but not much more.