I should've stated from an American hoops follower's point of view(NBA, NCAA, AAU only). Clearly there are plenty of folks on r/nba that follow European hoops and consume draft prospect ESPN Insider analysis, but none of my friends nor the hoops media I consume ever talked about Luka in 2015/early 2016. Once again this is from a layman's American-centric point of view, where the Olympics are the only time we really pay attention to hoops outside of the US.
These are the 5 links I could find on r/nba using redditsearch.io for r/nba posts talking about Luka Doncic b/w 1/1/2010 and 4/13/16.
It's not the best method but it looks like people weren't really talking about Luka back then on here. I couldn't find much on google from American websites(there was stuff on FIBA), but here's a Bleacher Report piece from late 2015 on Luka. Like I said I'm not a Euro hoops guy, and I'm not saying Amin is some genius, but clearly his take is still good. I didn't do a super thorough American NBA media twitter deep dive, but I'd imagine he's the most prominent one to tweet about Luka in early 2016 or prior to that.