Im more inclined to believe that Editch cavell the polyantha and not the HT was one of the parents. That would make it (diploid x diploid) x tetraploid = triploid. However, it is possible that (tetraploid x diploid) x tetraploid = triploid, but it is slightly less likely. Combine that with the fact that the HT is light yellow and it seems more probable that the red polyantha is the actual parent.