He's talking about the unit limit in the game, period. On huge maps like Faerun, especially with themes with auto-spawns, a limit is exceeded around mid-game - all of a sudden, scouting starts to work very erratically, you can't even always see the troops in a province neighboring your own.
At some point I understand the game can also crash, but haven't seen that - the invisible troop side effect has been bad enough for me to avoid large maps since knowing of it. ( As if the micro-management and 3 hour turns weren't bad enough
)