That line isn't a strong implication & doesn't require a stretch at all. It's just that you've become so used to interpreting it that way. Mind you I'd agree that it would be a strange thing to say if they'd met just 5 years earlier in Rebels. However if Vader & Obi-Wan crossed paths just a few years after RotS, & then they hadn't seen each other in 15 years it would be a completely reasonable thing for Vader to say. It's Vader bragging about all of his development in the past 15 years. I agree that it would be silly for them to meet during Rebels though. An encounter only 5 years before ANH wouldn't match up with that dialogue.