No, calling him an unranked dude in the context of the weight class they are fighting at, WHERE HE IS NOT RANKED, is 100% accurate. Yes, he was champ at LW, and lost the title about four and a half years ago. For reference, on that date, Khabib's most recent fight was against Darrell Horcher.
Right, it's accurate, but it's disingenuous. I don't get why you're putting down RDA. Dude is a well respected fighter who has been fighting the best of the best for the better part of a decade. It's not unusual for ranked guys to fight ranked guys when they change weight classes even if they "aren't ranked" in the new division, especially when they have some name value like RDA, especially, especially when they were the champ in that weight class.
Whether it was smart for Felder to take it on short notice, I don't have a strong opinion on it. If he won, he's right back in line with guys like Hooker and Oliveira to fight some of the bigger names like Gaethje, Tony, Poirier, unranked Chandler, etc. But at the same time, I don't really think he is championship caliber anyway. He's good and he's a tough SOB, but he's not an elite fighter. So if he gets a nice payday from the UFC and earns some respect from the UFC, other fighters, and the fans, who cares if he loses? If he wants to keep fighting he'll likely get someone in the #10-15 range...basically in the same spot as he was 9 months ago when he lost to Hooker.