Mate. Koby literally explains it and he's a member of the Navy with greater knowledge of the events as an outsider who lives in a closed off country with no intel on the Navy at all. Killer literally confirms in this chapter how difficult it is to get your hands on outside information.
Koby stated that they don't have the Manpower to use on the alliance because of everything explained prior. We're running circles here.
Who said Orochi has no knowledge? We know he knows about Vegapunk whereas Nami had to spy on the marines to find out about him.
Koby explains they don't have the manpower to take on an emperor crew who isn't even in an alliance.
Orochi tells us Wano can't fall because of Kaido.
The gorosei tell us they can't take over Wano even though they are injured and lost men.
The rational conclusion? The alliance>4 admirals and tens of thousands of marines.
Wouldn't he suprised if they could low diff marineford