Hang on: The Tee Spring code checks if GA is there and works without it, the website breaks because Ad block replaces GA with a fake object that doesn't have the same API. Perhaps some companies have the resources to check if their websites run with different ad blockers, but expecting websites to work when plugins are replacing and breaking running code is a big ask.
But you don't even need to check, you just don't blow up and still check out if it doesn't work? Put it this way, forget adblock, if the GA call fails, it should still check out. (Assuming the problem doesn't affect other stuff too, like internet down or whatever.)
Given the hyper-malleable nature of JavaScript in an HTML page in a user-agent owned by the end user, at some point the developer has to draw a line in the sand and say "This category of failure modes is not checked," because it's mathematically impossible to check all possible failure modes in that configuration. "An extension is intentionally faking an object in the `window` context" just happens to be on the other side of the "don't check" line for this application, because it's extremely unlikely (and, one could argue, user-self-inflicted).