I think he is correct in the assessment of the spring getting improperly heat treated. Maybe the level of heat treat works when the pull is less then a 9-10, but blows out under the higher stress.
I recently posted about some older Bokers that I found 2 examples of a spring breaking just like the ones in the video. Once I had them taken apart my inspection revealed that the parts did not completely “mesh” in the way SAKs and other knives do, and due to this there was a lot of internal stress built in.
In contrast I worked with two different older solingen made pruning knives that had bear trap pulls 9-10, and their springs were surviving, despite one also not having great fit between the blade and the spring.