Upon looking up crossing a triploid with a diploid, this came up:
"Therefore, triploids may produce tetraploid offspring when selfed or crossed to diploids or tetraploids. Because the probability of two 2n gametes uniting is expected to be low, researchers have hypothesized that triploids will play an important role in tetraploid formation and evolution (Harlan & de Wet, 1975)"
If the procimequat x poncirus created a tetraploid, maybe the greater genomic complexity could explain a palatable F1 with poncirus?
I guess the only way to know would be to ask the breeder about the siblings from this cross.