OK apparently that source I saw mixed up Lostbelt 6 with all of FGO lol.
Lostbelt 6 is a bit over half as long as Umineko in it's entirety, so I guess technically it's a bit longer than eithernthe Question or Answer arcs.
That's still pretty fuckin' nuts. And FGKnin general still being 4.5 times longer than Umineko is ALSO still pretty fuckin' nuts.
https://i.ibb.co/3M3Gmgj/r-CEGjv8-Q2-Stzp2rdtr9pg-UVi.png