It's perfectly reasonable given guilds were how many things were organized before a certain point in history. Even candlemakers may have their local candlemaker guild. For a fantasy setting, you just need it acknowledged having a guild for an occupation is common. Why wouldn't mercenaries have a guild that ensures the reputation of their own work? Otherwise, you're hiring anyone who happens by and hope they aren't just bandits or murderers. Only truly desperate people will hire some random swordsman and risk they vanish with the money or rob you for more. Mercenary bands known well by their name literally existed.

The rest is just a matter of LABELING. Don't like calling it an adventuer's guild? Name it literally anything else, then.