Hello chitgoks
Your assumption is incorrect. Only the Glacier Express (GEX), the Golden Pass Express (GPX), and the Gotthard Panorama Express (GoPex) do require a seat reservation. For all other PE trains, it is fully optional, so it is on the Lucerne Interlaken Express. The latter is a train that is daily used by the locals, too, for example, for going to work or shopping. Thus, a mandatory seat reservation would not be practical.
If you still want to make a seat reservation, you can. Yet, you have to do it with the operating railway, through this specific website:
https://shop.luzern.com/en/stories/luzern-interlaken-express
For a better understanding, I suggest reading our blog about seat reservations on Swiss trains:
https://community.sbb.ch/blog/23887-demystifying-seat-reservations-on-swiss-trains-a-comprehensive-guide
Kind regards,
Roland