I'd also prefer to see him in the front 3, but you're wrong, he definitely has CM potential. The reason it's being brought up is because in the event Alexis stays, it's the position Lemar would most likely take up (as Alexis and Özil would be tough to displace).Lemar is unequivically not a box-to-box midfielder, nor does he play that role. He's an attacking midfielder that spends most of his time on the left, but can also play as a no. 10. With Mendy a presence overlapping, he's shown the ability to tuck inside, but his penchant for left-footed crosses regularly offers great width. He's exactly the type of wide playmaker Arséne had in mind when we pursued Juan Mata. However, that's when Mata was 23. Lemar is only 21, has a much better work rate, and just tallied 14 goals and 17 assists.
Remember that in the 3-4-3, the more attacking CM has the ability to express himself farther forward, as there are more numbers behind him to provide defensive cover. Lemar has the passing, dribbling, and attacking ability to play this role well as we've seen many times this season, while also being surprisingly adept defensively. Per 90 mins, he has more completed more tackles and interceptions than Ramsey who has been considered a success next to Xhaka at CM this season. And to be fair, I used CL stats for Lemar where he faced tougher competition.
Beyond looking at stats, you can see in his play that he already pretty much plays the CM role for Monaco and now France. You'll very often catch him picking up the ball from CBs in the center to start an attack through a pass or dribble, and he often wanders around in a free role (often pretty centrally) to help build up play. I think it's beyond speculative, and even plausible, to assume he could play as a #8.
Again, I think he'd be best playing where Alexis and Özil are right now, but he definitely could play behind them so that the trio could be accommodated in the side all at once. He'd be able to play around the box in that role anyways.