I always saw Mac's "undershirt" to be one of those waffle-weave thermals. It doesn't seem near bulky enough to be an actual sweater. And I've seen a lot of waffle-weave thermals the last few years. They're no longer just considered "long underwear" like they were 20/30 years ago. So that would make sense... I even have a few waffle-weave tops myself.
As for the short, I thought it was good. And it was interesting seeing Mac without his trademark red shirt for a few seconds. As well as what's actually in the equally trademark backpack. I had seen it once before, but today I managed to record it. Yay!
I think it's one of my favorite shorts (although there's still....uh....I think 2 that I haven't seen yet).