The answer is yes, motors C and D do not respond.
I also note that the test script makes the motors change direction repeatedly, which might cause the current draw to spike? Leo’s actual robot scripts run at lower power and don’t change direction.
Leo tried running off just USB power supply initially because his BrickPi arrived without the AA battery holder (we ordered one separately from Amazon). I don’t think any damage was done there, because his first iteration of robot used the AA batteries and motors B & C. He chose B & C to make the wiring tidier. A photo of that iteration of the robot is attached.
(if attempting to run motors from a USB power supply might damage the Pi or BrickPi, you may want to make a prominent warning on the web page that you referenced)
Are the C and D motors controlled by a common circuit? And is that separate from A and B?
(thanks for the help)