I developed the first version of Google's equivalent of this (albeit theirs actually computes a constantly rotating key from the environment, it doesn't just hard-code it in the program!).
The reason it has to block until it's loaded is that otherwise the signal being missing doesn't imply automation. The user might have just typed before it loaded. If you know a legit user will always deliver the data, you can use the absence of it to infer something about what's happening on the client. You can obviously track metrics like "key event occurred before bot detection script did" without using it as an automation signal, just for monitoring.
The reason it has to block until it's loaded is that otherwise the signal being missing doesn't imply automation. The user might have just typed before it loaded. If you know a legit user will always deliver the data, you can use the absence of it to infer something about what's happening on the client. You can obviously track metrics like "key event occurred before bot detection script did" without using it as an automation signal, just for monitoring.