And the simple fact is that Adventurers did exist in our world at various times and places.

Maybe they went by Bounty Hunter or Explorer or they were scout an trappers and mountian men and filibusters and privateers, but they were people with Certain Set of Skills, working generally out on the fringes of "civilization" and doing jobs very much like hunting goblins.

But when you boil it down, they were people who hired out to do jobs that state actors like the army or navy or feudal levies or whatever either weren't available for or specialized to do or weren't seen as worth maintaining when they were only needed for a specific instance.

A government issuing letters of marque and reprisal to private aquatic murder hoboes to disrupt the shipping of an adversary in time of war is exactly a government (like Baron Whatshisname) hiring adventurers (but ones with a boat) to do military adjacent stuff that the regular navy was too busy or widely spread out to take care of.

So I cannot for the life of me see how any of this strains believability. History has shown us much weirder stuff.