Believe it or not, over in /r/zen some kind of sectarian conflict has almost broken out! In a nutshell - various users are adamant that Zen was, historically, an entirely separate, pre-existing line of thought that has been mislabelled/co-opted/misinterpreted as an offshoot of Buddhism.

Are there any historians of East Asian religion (or even general East Asian history) that can offer a definitive case for or against this assertion?