Very odd. I can find no reference to Sony's uniform having been designed by Issey Miyake, even in Japanese, other than this one anecdote.
Edit: There is some allusion to it, yet no direct reference .. However, it seems he was generally interested in using Japanese fabrics to make clothes for workers in the 1970s, so it is definitely possible.
I googled for a while in both languages and couldn't find one, unfortunately. I did find a book on Amazon Japan with Sony uniform pictures (amongst other companies), but I doubt the specific uniform here is included.
Edit: There is some allusion to it, yet no direct reference .. However, it seems he was generally interested in using Japanese fabrics to make clothes for workers in the 1970s, so it is definitely possible.