If you visit CyberMario's profile, you can view his upload from May 1st 2016 titled, "True Final Arena First Try Finish". In the description, you'll find a link to his Twitch channel, which reveals the following bio:

"Born in the wonderful year of 1993, I enjoy video games for the vast immersion and wonders."

So, either Nintendo decided to recruit this six year old employee in 1999, or this post reeks of "My Uncle works at Nintendo" - I'm going with the latter.

Besides that, exploring the area with clipping reveals that there is nothing on other side of the wall in Crystal Caves, so 'walling off' the content would have been very easy to find.

Well, that comment from CyberMario ends with, "From - sevinPackage" which makes me think he's copy pasting something from another user who says they've worked for Nintendo.

sevinPackage, stylized as such, does exist and is a game developer. Personal Twitter and sevinPackage Games Twitter. A quick skim can neither confirm nor deny that he worked with Nintendo or Rare, but I haven't the time at the moment to truly go digging.

https://mobile.twitter.com/sevinpackage
https://mobile.twitter.com/sevinPackageG

Not going to full on dox the guy, but I will confirm that I found he was a game tester in that era. There's a gap in his credits around the time that Tooie and DK64 were coming out, which may be because they didn't credit all the testers (or because it was one of his colleagues working on it)

Fair enough, though I will still reserve judgment on his direct involvement with Donkey Kong 64 for now.
The most telling part of the quote is the last sentence:

The room where the connection was going to happen still exists in DK64, but it's walled off and can't be accessed.

Technically, the chamber has been 'walled off', as the doorway to this secret chamber was replaced with a wall, but as far as the location still existing in the game... I have my doubts.

It's not as though we're looking at a new map as well - the chamber would have been a part of Crystal Caves, so if you load that area, you load in the geometry of the 'secret chamber' as well.

I might be coming off as overly skeptical here, but I've seen how this community has been burned with similar stories in the past.

His recollection is probably wrong tbh. Or maybe it was walled off for a bit before they decided to outright remove it.

I think it was too risky to left "connection chamber" in the game. Even walled-off GS hackers could access it and reveal dropped feature and went against Nintendo's order. At the same time GS hackers found a way to collect Ice Key in Banjo-Kazooie so it was very serious risk.