TBH I don't see this as a TeeSpring problem - the code on their page guards against a realistic scenario (the GA script fails to load), but the "break" is caused by uBlock's changes. The article's title is correct: ad block shouldn't break checkout. Injecting a bogus value into a global breaks checkout, so ad block shouldn't do that.
I agree with that, though if I'm TeeSpring, I'd fix it anyway. It's silly to throw out incoming business. I assume the same thing would happen if there were a brief GA outage.
The author is running an extension that specifically injects code to make window.ga into something unexpected, and then complaining about it.