Well, it's not against rules to note that a steam key doesn't exist for a given game, or that the game in question recently had a free beta key, or any other relevant details which may be useful to either entrants or the gifter. Thus, ultimately, you're allowed to provide information which offers the implication in question, you're just not allowed to state anything that's more direct than implication.

In many cases, allowing such comments leads rapidly to unnecessary toxicity, and in certain cases the circumstances are legit and a user is being harassed unfairly (and may even leave the site over it)- all the more so if the toxic factor also comes into play. Given that, staff tends to be pretty firm on that rule (though there is always a possibility of only getting a warning on your first violation, if staff feels it's suitable- for example, if they feel you're new enough to the site).

In conclusion, in many cases noone'll argue with you being right, as it's often rather obvious. In those cases, there's a lot of leniency on how heavily you can imply the matter. You just have to be careful of maintaing a certain level of respect (in the sense of not declaring anything directly against the user or their offering), and not stating anything that's not supported by evidence.