The firearms parts he was viewing were suggested to me as things people you know are interested in. He talks about it all the time, so I have him a hard time on the patriotic-sounding names to the parts.

That paper isnt dealing with the same issue that I am talking about. Learning chinese isn't going to compromise someones' ability to understand physics. (Unless the problem is written in chinese I guess) But being weak on unit conversion will absolutely compromise their capability to learn a concept where the first problems they are meant to solve are written in a way that requires unit conversions.

In my experience tutoring thousands of students, one of the most common scenarios that I see is that a student struggled in an area like that, was taught in a way that relies on that weak skill to learn the new skill when that was not necessary and then struggled with the new skill as a result. Thats the sort of scenario that this thread is about. Mixing in concepts unnecessary to understanding into a problem thus overcomplicating it before the student has a solid grasp of the concept. That does make it harder for them to learn and its something bad teachers do.