Well as a first point, I think that there's a huge difference between 'This very difficult thing is probably so difficult that X' and 'I have physically seen this production line, and there's no way it's doing X'.

The first is an estimation of difficulty - probably an educated guess that could be wrong. The second is a statement based on the evidence in front of you.

Secondly, whilst it's true that people have been saying Tesla won't get to 5k/w, it's also true that Tesla were claiming they'd have a fully automated production line. If Tesla gets to 5k/w by hand-building them all then in most people's eyes they've failed. The value of Tesla is not the actual production of cars. It's the IP of their self-driving technology, and the IP for their fully automated production line. There's a reason Tesla's valuation is completely unlinked from their production capacity.