If Flacco slides earlier, of course it would be moot. The hit wouldn't have happened. I've essentially already mentioned the hit was intentional. "...no matter when Flacco slides." Really? So you're saying that if Flacco slides five yards, ten yards, sooner Alonzo would've still cleaned his clock? That's silly.
If it's not Flacco's responsibility to slide, who's is it? He was the QB running the ball on the play. To slide or not to slide? That is HIS question to answer. Nobody else on the field is responsible for that. Nobody.
Whether or not it would've been a "weak move" is a subjective matter. Objectively, the Ravens' player hit Alonzo in the head. He did so with enough force to knock Alonzo's helmet off. That, by rule, is a personal foul. If you want to throw a flag at Suh as well for his part, I can't argue against that.
From what I saw, nobody got that play correct. Players, refs, nobody...