I think Rocco's comment was in there simply to strengthen our suspicions at the time that Pentangelli was behind the hit attempt.

Michael meets with Ola...everything is fine.

Then he argues with Pentangelli.

Naturally, after the hit attempt, who do we suspect?

Then the meeting at Pentangelli's house...."In my home...in my bedroom...."

Then Michael hits Frankie (and us) with his surpise bombshell, much to Frankie's relief:

"It was Roth."