Sure he was. And a dangerous one at that. While they are roughly pretty good at helping us figure out the important names in a division, rankings are not the be-all end-all when deciding who's a contender and who's not. The guy ranked number 5, while pretty damn good, may not be as good as number 12; he might even have a loss against him. It's the quality of wins, both in terms of opposition and style points, that makes you a legit contender. Thiago had over 15 fights in the UFC by the time he fought Anders and scored clean, violent KO's over guys like Hermansson, Lionheart, Marshman, and Meerschaerdt. There were a bunch of other guys he flatlined or beat convincingly but those would be the big ones. Of course, he also had losses to Eric Spicely and David Branch. Mousasi did spark him too. Hmmm. I'll downgrade him to inconsistent Just Bleed disciple at the time of the Anders fight, but I won't accept classifying him as a journeyman; he's always been way more talented than that (based on my personal definition of what a journeyman is). Regardless, he was more popular with the general public than Islam is now, Anders was way more well-recognized than Moises ever will be (again, already headlined a main event), and the fight was guaranteed to be a banger so I still maintain this was a better main event than Makhachev/Moises by a mile.
For the record, no one that knew what Marreta was capable of was all that surprised at Santos' success at LHW. The guy was a huge MW that could push the pace for as long as the fight lasted, had very good TDD pre-knee surgery (not that there are a lot of monster grapplers at 205 these days), was fast as hell, and had the kind of power that would translate well at HW, never mind 205. Besides Rumble I can not think of a harder hitting non-HW in the history of the sport. Maybe Manhoef? Seriously, the guy could punch or kick (or wheelkick) holes in teflon and the variety of techniques he could get you out of there is truly impressive. His downside has always been his glass chin and to a lesser degree his sub defense, but in a division full of far slower kickboxers it stood to reason Thiago would do well there.
That's my take anyways.