Yes I think you are correct. I have successfully loaded and run the Pai - Paignton to Taunton scenario, but I do have the Class 143 and the JT Class 220. These are trains that pass on the other line.
I have run into a similar problem with Western Mainline. It is one of the downsides of Just Trains add-ons that you need extra locos to run the scenarios - as it comes the WML can only effectively run 3 scenarios without purchasing extra locos. Hence the costs are far higher than quoted for the Add-ons, far more so than any add-ons from Dovetail.
The Just Train add-ons are excellent quality when they work, which they do most of the time if you are happy to purchase the extra locos. There are software bugs in the software though, WML plus both Paignton extension and Oxford-Bicester extension, which fortunately only affect a couple of scenarios. Probably, on balance, just about worth the extra costs.
The Class 143 and the JT Class 220 are both excellent and fun locos to drive so glad I invested. The HST is also one of my favourite diesel locos so have fun with the scenario.