What's the subpatellar tendon?
I suspect that you are correct, "subpatellar tendon" is referring to the patellar ligament. Although it is small, in the image below, (3) is the tibial tuberosity and (5) is the patella. (4) which is labeled "subpatellar tendon" is between the two. The only structure between them is the patellar ligament. So, I think this is what it meant (though in 20 years of teaching, I haven't heard that terminology).