With Exum it was different. Exum has elite quickness and size - it was a question of him learning proper technique and desire to defend. Lyles doesn't have elite measurables. Exum's D was a question simply because he never had to defend at a serious level, not because his D has been tested and proven to be unable to defend. With Lyles it has been tested and it has been less than stellar. He isn't fast enough to guard 3 and he hasn't shown ability(or willingness) to fight for position in the post defensively.
Also, citing the exceptions to the rule is not really a good way to predict the future success of a prospect. You'd have better success by going with the majority of players that have had similar success defending in college. Of course he can improve tremendously and some day become a DPOY candidate, but lets be honest with ourselves - what do we really think is the chance of that happening?