You are committing the fallacy of confusing the origins (and you are wrong on that too) of something with its truthfulness.
This is a variety of ad hominen attack. Discredit the person and subsequently cast a shadow over everything they do.
The Nazi's and all bad men use a variety of tools to perpetuate their power.
If it is shown that somatotype is an accurate predictor of temperament that fact would stand whether it was used as a tool to manipulate people.
As of now you could say the same thing about personality tests. They are routinely used to hire,fire, promote, and put people on the fast track. Same with IQ tests. Is there something that IQ tests measure? And are those measurements used by certain people to make decisions about peoples future income and happiness. Yes. Indeed.
So your knee-jerk reaction pretty much helps me size up the current state of YOUR ability to carry out an objective investigation of this subject and probably most subjects.