I’ve always viewed it as:
There are canonical deaths in Hitman. Based on conversations from NPC’s and general themes, canonically, Agent 47 always gets Silent Assassin (not suit only, the games are unique in their social stealth and the very first training mission points out that nobody has ever done it before).
In almost all cases, there are canonical ways that characters die. These are almost all entirely accidents, which is Agent 47’s M.O.
So then, why are we able to play however we want? Because it’s a game! More specifically: The Story Missions reward map and target knowledge. Through the course of Mastery, through the course of saves and reloads, players will become very familiar with what makes the game “tick.” If you already know where your target is going to be, you get more points for doing the same kill faster.
Exploration, experimentation, and multiple playthroughs are rewarded! And while killing everybody on the map results in 0 points, this is still a “success.” 0 points still goes on the leaderboard.
Doing a “kill everybody” might not be canonical (insert two Colorado guards talking about Sapienza being a ghost town). However, the game ultimately rewards mission knowledge, which means it rewards experimentation and exploration. Killing the entire map might reveal unique or new information. And this is ultimately what WoA wants you to discover! However you do that is up to you.