I've seen people bring up that line before as if it's evidence of Kenobi encountering Vader post-Mustafar, but it really isn't, and for one very simple reason: context!
That line is delivered in RotJ, AFTER Kenobi faced Vader on the Death Star and achieved oneness with the universe. Of course he knows he's 1) alive and 2) a cyborg *because we saw them fight.*
Ah right, I thought I was forgetting something about why the line worked without a post-Mustafar encounter. I don't think there's anything in the movies that contradicts such an encounter, but I get why it works better in the broader story for Obi-Wan and Vader not to meet between ROTS and ANH.
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