I just went through this, if you try it on the steam client it will report the key as valid (I have no idea why it won't work on Mobile), thats something maybe they can answer, maybe not.
Fact remains, the steam client will accept it as a valid key.
I can't see any good reason for someone to be posting bad keys...what would you gain? A few likes?
And even if someone were, wouldn't it make more sense to post already claimed keys from anywhere (even here) instead of invalid ones?
You can gain likes by simply posting as I am now or saying hello lol...try the PC steam client and NOW i'm sure it will say the key has been claimed.
I don't understand the reasoning of why the pc client is required, but I also don't believe they are fake keys that happen to be accepted on PC and not mobile, that would be harder to do than its worth.
Figured before this turned into a repeat of history, I'd try and clear it up
Besides, it was a FREE key, don't get so worked up....like you payed or traded for it or something heh. No skin off your back.