What an absolute crock of !@#$%^& that video is.
By 1940 not just DAC but most, if not all, US aircraft companies were using hydroforming (that is the common aviation industry name for it) and the video has it backwards, In the old tech both the form block and the punch were often steel. With hydroforming production form blocks can be cast then machined to shape. Prototyping and limited production form blocks are made of wood and good for multiple components. Any changes or errors etc and the cast block can be melted down and reused. Only the punch was rubber and its flows are highly predictable. AND, the process is still in use today and is so easy that even I have used it.
If it created random unpredictable stresses like that video suggests, the aircraft would develop stress cracking and start breaking up within hours of completion. Instead there are DC-2s and DC-3s and a "gazillion" other aircraft still flying that were built using this tech.
Somewhere I have a PDF USN basic primer on how hydroforming works dated early 1940's and many of the other texts of the time cover this practice.. I will try and find it and post it. I also have somewhere some pre ww2 photos of NAA form blocks for hydroforming parts for the BC-1 aircraft.
The following page is from the 1944 publication
Aircraft tooling practices, by James R. Yeakley. If necessary I will complete scanning that and post that instead.