No one should doubt Vergeben is the real deal by now. If he heard Minecraft content was going to be in Smash then I believe he legitimately did. However I've been saying that Minecraft could have been a code word used for Banjo for sometime now and I'm sticking to my idea. In fact here's a theory I'm putting forth in all this.. this is heaily playing off of
papagenos
theory
Nintendo contacted companies for 3rd parties and Microsoft was one of them. They probably asked about Banjo, there may have been some video conferences or something between higher up's here and there discussing the possibility of Banjos inclusion due to fan demand. A handshake deal happen's where MS says yes to Banjo but there's still legal matters, signatures and what not to deal with possibly even the fact that Banjos copyright would need to be renewed.
Sakurai won't do actual work on Banjo until everything is squared away BUT does start the planning process, it could be at this point that the character is referred to as "Minecraft" to throw off potential leakers. Keep in mind here that for the whole 3 years of Smash Ults development we knew nothing about it. So Nintendo did do well with containing leaks.. we didn't know even about all characters returning and what not until E3 hit and people started seeing promo material. Internally Sakurai did well to keep things hush hush so using a code word for a highly requested 3rd party wouldn't shock me.
Anyway so Minecraft could have been the term used amongst Sakurais staff when referring to anything Banjo related. This could be why leakers heard about it.
E3 2018 role's around and Sakurai/Nintendo as well as Phil Spencer/Rare go to lunch with one another to finalize the Banjo usage. This could have been out of convenience since they'd both be in the same place at the same time. I imagine it would have taken Nintendo and MS so long to meet because I'm sure they probably prioritized Banjo last after all the Japanese 3rd parties were on board.
Anyway the deal could have been completed there, which then Sakurai hurries back to Japan and attempts to push Banjo into the base game even moving Pirahana Plant to DLC in an attempt to get him in. After a few months though it could have been obvious to him that Banjo isn't going to be completed by then. This is probably when he decided to move Banjo to DLC and even scrub any Rare stuff out as to not reveal their collaboration. I'd even go as far as to say 5 characters weren't originally going to be DLC and instead we'd have received 4. Banjo could have been tacked on before everything was finalised and of course PP became a "free" DLC character mostly due to him being incomplete from being moved out of priority but since Sakurai wanted him for the base roster and him being so close to completion he decided to make him a bonus.
This brings us to where we are now... obviously I could be 150% wrong and that's fine... I'm not an authority on anything nor do I have any knowledge of the development of Ultimate. Banjo could be none existant for all I know. It's just a theory/speculation on my part.