It is a bit tricky with Day Passes and Saver Day Passes. Both allow free travel on all routes where the GA Travel Card is granting free travel, too. As a general saying, free travel is possible to inhabited places where people live 365 days yet not beyond. It is different for routes where the GA travel card does not grant free travel yet a discount.
The Day Passes and Saver Day Passes do not grant any discounts on routes beyond inhabited places (e.g. Wengen), for example, on mountain railways or cable cars. There are a few exceptions though, being Mt. Rigi, Mt. Stanserhorn, Stoss, Klewenalp / Stockhütte, and Mt. Rochers-de-Naye. Elsewhere, a full fare Day Pass or a full fare Saver Day Pass is not valid and does not give any discount. What is decisive to receiving a discount with these tickets, is whether you hold, in addition, a (Swiss) Half Fare Card or not. If you hold a Half Fare Card, it is actually the (Swiss) Half Fare Card that grants the discount, not the (Saver) Day Pass.
The ticket checker was very friendly as the person only asked for a supplement of CHF 10, being well aware that you misunderstood the reduction rule. It is actually not a fine, which would have resulted in a CHF 90 surcharge plus a new ticket.