tanky76
The Swiss Travel Pass 25% discount on Jungfrau area tickets instead of the usual 50% is a quite unique exception specific to the Jungfrau Railways, and the SBB Mobile app does not support this unusual 25% discount category for calculating ticket prices.
Therefore, with a Swiss Travel Pass you should get those tickets only via the jungfrau.ch ticket webshop (or buy them on site at the Grindelwald Terminal).
Your assumption/question in the topic title is correct though for most other (if not all) mountain railway/cableway operators who are granting 50% on GA/half fare cards and the STP.