Well, I must say that I am not 100% sure if it actually worked with the Wifi. It did change the MAC-address for the network card en0, which I recently realized is the ethernet card. And I did have the ethernet cable connected earlier. Only later I realized I was not observing it changing the en1 (Wifi) MAC so I don't actually know if it worked with the Wifi or not at any time.

So, IMO it can be one of the following:

WifiSpoof never worked with late 2009 (or late 2010) iMac Wifi card it did but Wifispoof update caused it to stop working or updating Mac OS Sierra to High Sierra + possibly one or both things above = not compatible