You are talking about the Matterhorn Glacier Paradise offer, not to be confused with the Glacier Express, the panoramic train from Zermatt to St. Moritz. Furthermore, you are referring to the information given on this page: https://www.matterhornparadise.ch/en/Book/Online-tickets/Single-tickets-Matterhorn-Glacier-Paradise
Using the SBB Mobile app, or having the Swiss Travel Pass as PDF on your phone, you cannot buy discounted tickets online for the Matterhorn Glacier Paradise. You must buy them on-site.
One can buy it online provided they hold a Swiss Pass card, which is primarily used by residents only. It is a card that has an RFID chip in it, on which the tickets will be stored.
For you as a tourist, you can buy it on-site, as stated. And, I suggest you do not buy it too far in advance. The weather plays an important role in our country; in case you cannot or do not want to visit the Matterhorn Glacier Paradise due to bad weather, you will have to see how you get a refund for your prepurchased tickets. Again, buy it on-site once you have a clear picture of the weather situation.