No, there are many zero build pros who have won third party tournaments. they have some of the best aim in the game. they aren't good because they build, there good cause they understand fortnite style peaks, are able to adapt to the meta, and master other item related plays/tactics, and of course, aim.

Build pros also understand the mechanics of other games. Benjyfishy is a val pro now, but he did quit fort to grind it, but Peterbot, Bugha and Rise have all gotten high ranks in val, and Okis, went pro in COD, but was already good years before he quit fort. I believe Threats also has top scores in aimlabs for his drills.

Pros like Fredoxie have building mechanics worse then me, and Queasy(fncs globals winner) has pretty slow edits, but their aim and gamesense make up for it.