In the US, an open source clothing design is superfluous since such designs are not usually protected (outside the scope of copyright, rarely qualify for patent, trade dress, or other protection.)
Clothing design in the copyright context means the pattern of the clothing, not images or text, and copyright for clothing generally applies to non-functional flourishes or design elements. In this case, the logos are derivative works of presumably copyrighted logos of open source projects.
Im repeating myself, but have a look at freesewing.org, it's an open source platform for made-to-measure sewing patterns.
Full disclosure: I'm the maintainer
What do you mean? There are actual print designs for t-shirt print shops!
They probably mean instructions to create a t-shirt from raw materials.