When Roth says this to Michael about the hit on Moe Greene, isn’t obvious that everyone knew it was Michael that gave the order? He killed not only Moe but the heads of the five families also all at once! It’s not hard to put two and two together to figure out what went down.

Absolutely. Mike wants everyone to know but at the same time they wouldn’t dare say it openly

Of course Roth knew it was Michael. His emphasis was in the last thing he said in his statement..."it [Moe's murder] had nothing to do with business," meaning that business (including his business with Michael) was more important than his personal feelings for Moe. He was chiding Michael for questioning the attack on Pentangeli--acting like the wise elder stateman to pressure Michael to cough up the $2 million.

It's one of the most brilliant scenes in the Trilogy, and probably earned Strasberg his Oscar.

It was a great scene. Roth's smouldering resentment over Moe was palpable. You (well, at least I) get the sense that he had been planning his revenge ever since. It makes you (well, at least me) wonder how he came up with such a ham-fisted plan.