I think it was just one more scene showing Fredo being inept. Also it seems as if every scene with Fredo he is either treated with no respect or as if he’s invisible. During the hit on Vito the two shooters were so unimpressed at the sight of Fredo with a gun, that they didn’t even look his way. I’m sure Solozzo would have told them not to harm Fredo if he was there because there was going to be enough bad blood with just Vito getting whacked. In GF II, while Frankie is happy to see Fredo, Willie Cicci seems pretty dismissive of Fredo. We never see any scene where Clemenza or Tessio acknowledge Fredo as someone important and deserving any notice. Moe Greene slaps him around. Tom treats him like he’s not there. Its no wonder that Fredo is upset during the boathouse scene with Michael when he says that he wants to be treated with respect.