I agree, that sewing plywood is rare, but I know it was used for some panels in the Vintage MB sports vehicles. I have got this couple of references too:
https://www.icd.uni-stuttgart.de/tea...-bent-plywood/
https://materialdistrict.com/article/sewing-wood/
The video above that show a 201 sewing plywood is mostly a statement about what the machine is able to do with sufficient driving punching power. The video channel show other examples too. I think the Singer 15 can be modified to sew plywood or other heavy materials as well. But the potted motor will be insufficient for that.
Until I see some new data, I stick to my the statement about the potted motor is weaker than other typical sewing machine motors. I guess that some people are convinced by dreams and others by figures and tests.
A lot of people love their Singer 15-91, and this machine can most likely do more heavy stuff that most domestic machines in general. The newer domestic plastic machines are no match to the good vintage machines.