This list is not about who is the most skilled, or best duelist, or best spitwad maker... This list is about the players that are consistently lethal.
Allers is a lethal player but he mostly goofs off so that is likely why he did not make the list.
You can argue all you want about goretooth for example, but at the end of the day he nets kills. Sure he is gaining a lot of Grace from his armour (even though when he is wearing his green pezzy outfit instead of his black armour he still is on the top five) sure he is using a build more suited for battle then for duels, but at the end of the day this list is about a watchlist not some arbitrary better-duelist-then-you.
Rhade for example was pure murder in a duel and is honestly one of the best duelists I have ever seen, but he was taken down faster then usual in battle due to his hyperspecialisation on dueling and his past dueling experience, and thus would have a problem of battlefield awareness and often get killed due to it.
So again, this is not a list for most skilled with X, this is a list for battle players that will kill you more often then not, or take down a huge swath of players before going down. Too many duelists have tunnel vision in battle or don't use proper tactics and terrain advantage, and the best skills of blocking won't save you there.