"it's completely doable to write [Pac-Man] in a purely functional style. The dependencies can be worked out; the data flow isn't really that bad. It still may be a messy endeavor, with lots of little bits of data to keep track of, and selectively moving parts out of the purely functional world will result in more manageable code. Now the obvious target is either the state of Pac-Man himself or the ghosts, but those are part of the core data flow of the program. Make those globally accessible and modifiable and all of a sudden a large part of the code has shifted from imperative to functional...and that wasn't the goal."
meaning that trying to make Pac-Man or ghosts into pieces of mutable state will result in them being manipulated entirely by being passed between functions, an even more functionally tangled program (in terms of less being able do some modifications to it as easily as could be done in a similar imperative program.)
Instead he's suggesting adding tiny self-managed globals for little values that truly are global (used almost everywhere) but aren't constant. Basically segregating tiny pieces of mutable state into their own little cubbyholes in order to simplify the logic.
Or at least that's how I'm reading it.