Manufacturing isn't defined by the assembly line, of course. Good data is hard to find, so take from it what you will, but the internet loosely suggests that only around 30% of manufacturing jobs are on the assembly line or adjacent to an assembly line. Anecdotal observation aligns with that, so I expect it is in the right ballpark.
> I don't think I've ever heard it
It is not so much the hot topic it once was, but when manufacturing was really in its decline you would frequently see in the news interviews with former manufacturing labour expressing such things as they lamented no longer being able to work in the industry.
It may not be sipping margaritas on the beach enjoyment, but on the spectrum it is unsurprising that many find it to be more enjoyable than other types of jobs. For as bad as you can imagine the assembly line to be, there is undoubtedly someone doing a job that is far worse.
> That's an ignorant, condescending description of assembly line work.
If these kinds of feelings are flooding your head, you have not considered the statement logically. That is not in good faith. Rationally, where do you find error in the statement?
> American companies can't find enough people with those skills as it is; there is no need for more of those jobs.
There is definitely an information problem. Manufacturing by and large happens in small town/rural areas (70% of it, according to the BLS), while people by and large live in large urban areas. The urban dwellers exclaim "Where are the jobs???" and the rural dwellers exclaim "Where are the workers???" It is a fascinating disconnect – something we see outside of manufacturing too.
You are right that this isn't apt to fix that problem. However, it is important to remember that the people calling for this aren't running complex mathematical models to ensure that moving manufacturing to the USA will be better for the world or whatever you think should be driving. They are simply in the mindset of: "I think I want to work in manufacturing. Give me that!" It is not a solution with well-considered grounding. It is an attempt to appease emotions.