Chiarelli referenced a potential offer sheet as a reason for the Eberle trade in the summer, I think sometime shortly after he pulled the trigger. What is interesting is that, earlier in the summer, before he made the Eberle trade, when asked about a potential offer sheet for Draisaitl, Chiarelli seemed surprised at the proposition, as if he was caught off guard by the possibility of someone even considering sending an offer sheet.
But, like you said, later on he did reference that as a reason for the trade, and he was pretty definitive about it. I tend to think that one of two things happened: 1.) Chiarelli thought that an offer sheet was a really convenient reason for the trade, perhaps filling the gap of the more awkward and probably true reason of Eberle asking for a trade; 2.) someone really was planning on sending an offer sheet, perhaps telling Chiarelli that if he doesn't free up space, then they will offer sheet Draisaitl. Bear in mind there were whispers of a looming Draisaitl offer sheet before he signed his contract.
Neither of these scenarios may be true, but I definitely noticed the discrepancy between Chiarelli's surprise at the possibility of an offer sheet to later citing it as a reason for making the Eberle trade.