What would it take to convince you, more pictures?
Of course a photo that hasn't been artistically altered would be convincing. It just seems clear that the delta shield has been a recurring theme for the film's promotion -- these posters remind us of that. The subtle delta shield pattern, as seen in this posters, could be just an "easter egg" of sorts -- a little detail for us to catch, just like the delta reflected in their eyes. The graphic designers might also have decided that the uniforms needed some sort of texture when printed in monochrome on large posters. At this point, the delta pattern could just be an artistic addition.
It does seem like, well... like a rather extravagant expense for something that the audience really couldn't give a flying @#$* about.
It's really not all that common for a film production to commission their own fabric production facility with their own custom weaving mechanics. You can do this with molding, or embossing, easily enough... but not with fabric weaving.
1) Maybe they spend half of the movie's budget setting up a specialized textile manufacturing facility?
2) Maybe they found a fabric already in production which has a "delta-shield-like" element in the weave already?
3) Maybe the overshirts aren't made from any fabric in the sense we think of (ie, they're molded plastic)?
4) Maybe the thing is simply photoshopped in, as is being proposed.
I find #1 to be INCREDIBLY UNLIKELY. And #3 brings in a whole new set of problems to be dealth with (fabric is easy to work with and looks good... other materials, less o).
So, for me, I tend to think that it's either #2, or #4. And if it's #2, that's just a case of dumb luck... of someone having stumbled upon this fabric.