Being "serious" is exactly what Floyd did. He CAME at Conor like no one else and he was getting legit counter punched. You can't fake being counter punched. Conor has been making people miss his whole career, Floyd even admitted he was surprised by it.
I'll explain to you the reason this is smart. Floyd knew Conor was dangerous the first few rounds because of his mma fights, he has decent power in his hands, but he has no cardio, especially for boxing. I'm not sure what "being serious" means to you, but being risky is not smart, and floyd isn't dumb, at all.
Conor is a real counter puncher though. If his cardio would have been decent(it was terrible) he would have gone to scorecard and lost, but that would have been a huge blow to boxing.