Yeah, I definitely don't agree with this.
Ban would gain nothing from stealing from King. Unlike at Edinburgh and with Galan and Mel, a serious King wouldn't be getting up close and personal with Ban and I doubt Ban would have a moments peace to actually use the technique. And if he did, the most King would lose would be the sharpness of his moves, since he's not even wearing armor anymore. But it wouldn't be instantaneous. With Physical Hunt, King and Diane weren't even paying attention or aware of Ban's shenanigans, so he had plenty of time to sit back and take everything they had. In Vaizel he had to take it slowly during the fight with Mel, but Mel still had plenty of time to keep fighting before Ban was even able to finish.
Both Ban and King struggled against fodder knights before, but given what we've seen now, it doesn't make sense for them to do so. Ban struggled against Jericho and Guila, but that obviously shouldn't have happened because we know that, even without a weapon, Ban could instantly stomp them. Diane struggling against the vampires makes sense because her key attribute was her physical strength at that point in time. King struggling doesn't make sense because he doesn't actually fight with his physical power, he fights with magic. If Physical Hunt/Hunter's Fest stole magic, it'd be clear that Ban would be King's superior, but that's not the case. I don't know what Nakaba was drinking during the creation of Edinburgh, but a lot of crap goes against what's been further established in the series.
Do I think Ban would have a chance to beat King? Yeah. I don't think it'd be that easy though. Sapping strength isn't going to be the instant win button for Ban just like Fossilization wouldn't be the instant win button for King who, might I add, was also basically holding back in the City of the Dead. We know what he's fully capable of. Ban let him capture him, sure, but there is nothing saying that King wouldn't have eventually hit him with Fossilization should they have really gone at it.