I think that's all false. Someone just mentioning something being open source doesn't mean you can assume the terms and materials applied. Being open source explicitly means it can not be forked and used in any way, only in ways that comply with the terms that make it open source in the first place. And instructables.com does not seem to apply any default license nor take ownership of posted content, so someone can post anything under any (or no) license there.
I agree that the "open source" designation alone does not define clear usage terms; these will need to be set out in a specific license. But again, which rights and what protection does Mr. Carlson hold for his design, i.e. to what extent
can he legally restrict usage rights?
In my understanding, the only protection is via copyright. Which protects Carlson's creations in their specific form: Instruction text and pictures, videos, original design drawings and layouts. He is free to state that these works cannot be used in conjunction with a commercial product or kit.
But if someone creates their own PCB layout and instructions based on Carlson's circuit, they are free to do with those what they like, including commercial use. The functional circuit alone, or the idea to put everything into a cigar tube, are not protected. (This type of technical solution
could be protected in principle, via a patent, if it were new and inventive
and if Carlson had filed a patent application before publishing. Both of which do not apply here to my knowledge.)