It sounds like the video is saying (link with a time stamp corresponds the following)
Wienert revealed that during one episode he personally assembled a Polynesian-style bamboo raft off-camera only for Grylls to add the finishing touches and take the credit.
cambridge dictionary gives this explanation about personally
If you do something personally, you do it yourself rather than asking someone else to do it
So, "personally" indicates that Wienert did the job himself, while not necessarily did that on his own. "personally" is being used here justify Wienert went through the whole procedure. is my understanding right?