I just took a look at the relevant page and I believe he's referring not to external reloads, which could be transfered to the internal tubes, but to extra tubes mounted outside the pressure hull. These were in some British classes and could be fired once but there was no way to get to them from the boat, even surfaced.
Here is the 'T' class submarine
HMS Thorn.
And HMS
Triumph after hitting a mine in 1939. Fortunately none of the torpedo warheads were set off by the explosion.