It's not like more than 0.1% of people visiting the page will see the noscript version
What makes you say this? Of the folks I know who might like to use a site about git, most of them use Firefox with Noscript. I'm sure my sample is biased, but I don't think you can assume it will never be more than 0.1%.
I thought he was making the general case, why a Mac picture at all as a choice, not why show a Mac specifically for the noscript.
If he means the latter, then, well, it doesn't matter at all. It's not like more than 0.1% of people visiting the page will see the noscript version --and it's not like anywhere should even care about what they will see.