Yes and no. If you charge the battery just until the charger indicates that it is done with the fast charge, you will have charged for 15 minutes or less. A reasonable person that hadn't read the manual, (like you and me), would think that it is completely done. In fact, it is only changing from the high charge rate to a trickle charge. If you stop charging at this point, you will get about 90% of full performance out of the battery. If that is all you need, then there is no reason not to just do a 15 minute charge. To get the last bit of charge into the battery, however, you will have to let it remain on the charger for another hour or a little more. It is still pretty remarkable that you can get a nearly fully charged battery in only 15 minutes.
The reason for the slight undercharge is that the battery gets hot from having charge crammed into it in such a short period of time. The charging current is over 8 Amps. That's why the 15 minute chargers actually blow cooling air over the battery and the charger's electronics.
Hot NiMh batteries do not hold as much charge as a room temperature battery. So it is necessary to allow the battery to cool down a bit before the last bit of charge can be put into the battery using a trickle charge of a fraction of an Amp.