for the hell of it. Here's a formula that relates the original torque to the new torque when an extension is used:

NT = OT/(1 + (b/a)cos(180 - theta)) with constraint a > b.

OT is the original torque, a is the torque wrench length from center-to-center, b is the extension length from center-to-center and NT is the new torque.

Using the diagram of torque wrench from the web site, when theta = 90, NT = OT. So their claim is true regardless of extension length.

For the case when theta = 180 degrees, i.e., when the extension is in line with the length of torque wrench then,

NT = OT/(1 + b/a)

As an example, when I do valve adjustment, the excentric locknut need be torqued 7 lb ft. So using an open ended 10 mm wrench of 0.35 ft long and torque wrench length of 1.125 ft, the torque wrench needs to read 5.32 lb ft to get a torque of 7 lb ft at the locknut.

So when the 90 degree angle method becomes awkward for those hard to reach places, then you can use the simple expression above to figure what torque is when the ext is in line with torque wrench.

Just trying to help a bit more.