You are abseloutly right, they are using GPON. but up untill now they had a package of 1GB so the media converter (TP-Link MC220L) would work together with the SFP module that you could buy from them.That one only supports 1gb. 2.5g is rather rare since 10g costs about the same and uses the same fiber. There tends to not be a reason to run fiber at the slower rate.
What I suspect though is the fiber connection you are calling 2.5g is not using ethernet so you can't use a simple media converter to begin with. The devices you talk about are primarily used by private individuals who own all the equipment including the fiber
Almost all internet connections are using GPON. You need a modem or a router that supports this....in many cases you need very specific ones compatible with your ISP because there are many variations of GPON. GPON is very different than fiber based ethernet. GPON only uses a single strand of fiber and that fiber is actually shared between users. Fiber ethernet uses a pair of fiber that are dedicated to a single user and are point to point.