"Er hat den Schlüssel aktiviert."
One doesn't typically activate a key... The proper German word for turning a key would be "drehen."
Yes, I definitely agree. When I read the German sentence I thought of some kind of electronic key that needs activation before you can use it... but then it simply said "He turned the key". drehen and umdrehen are much better for turning a key.
If you are talking about a software licence key or similar then it makes perfect sense.
Yes, that's how I interpreted it. Which is why I was a bit baffled when the alternative solution was supposed to be "He turned the key." - that translation is simply wrong.