Hi @b24, welcome to the forum 
As @nurjan14 listed right after that quote, there are non-inertial sensors that can provide a position estimate relative to some external source of information (e.g. sonar pings bouncing off the surroundings, or GPS signals, etc).
Not for any meaningful length of time. There have been some discussions around mixing in feed-forward control estimates like the estimated acceleration due to thruster actuation, which could help, but that is not yet implemented and hasn’t been tested.
Beyond that, this comment is the most complete description I’m aware of discussing ArduSub’s positioning abilities and requirements, especially as related to the prospects of IMU-based dead reckoning.