The script doesn't give enough evidence to make a definitive pronunciation on the matter, even though it seems largely intuitive that Connie would have known for the reasons already enunciated: the scene doesn't appear to be written to inspire trust in her words. Besides, throughout the whole series characters talk out of both sides of their mouths, so any narration most of them utter is prima facie unreliable, including from Connie (only what we would expect from someone who could as she did organize and even carry out extra-legal executions and not get caught). It almost defies reason that she would have discerned who was responsible for Carlo's death and not for Fredo's. Connie's personality does have a bit of mystery about it and at the beginning of the first film she had an air of naïve innocence about her but she lost her naïvety during the first film and her innocence was clearly gone by the second, and she certainly was never a "bimbo," even when at the beginning of part II she was pretending to be one.
And there is nothing implausible about her forgiving Michael so quickly for Fredo when it had taken so many years for her to get over Carlo. First of all, by the end of part II it is obvious Connie had changed a great deal and was much more resigned to Michael's management of the family:
"Michael, I hated you for so many years. I think that I did things to myself, to hurt myself so that you'd know - that I could hurt you. You were just being strong for all of us the way Papa was. And I forgive you. Can't you forgive Fredo? He's so sweet and helpless without you. You need me, Michael. I want to take care of you now."
Note as well that even though her pretext for offering to come back was to plead Fredo's case, she didn't make Michael's granting of it a condition for her return.
Second, by the end of part III she was so resigned as to kill her own godfather when it would pre-emptively help Michael's cause.
However, the strongest indication that the scriptwriters had in mind that Connie did know may be literary. For this we have to turn to the novel. The novel may not be canon for the films but it is instructive about Puzo's [and collaborator Coppola's] state of mind and attitudes about the characters and the universe. In the novel, both Connie and Kay quickly understood that Michael had made Connie a widow and were both livid at first but ended up forgiving him for it. Both, and - this is important - Kay very specifically, ended up resigning themselves to this sort of deed as terrible but necessary and as a secret but an open one. Kay leaves Michael at first, and then when Tom comes up to get her, he feeds her this obvious chicken dance on the subjet:
"You have no way of really knowing that's [Michael's killing Carlo is] all true. But just for the sake of argument let's assume that it's true. I'm not saying it is, remember. But what if I gave you what might be some justification for what he did. Or rather some possible justifications?"
Kay calls Tom out on this legal-ese and says, "It's not your best side," but in the end she accepts his justifications, at least enough to return to Michael.
In the movies, Connie and Kay understand quickly that Michael had killed Carlo but neither is resigned so quickly. Connie stays angry until near the end of part II and Kay tries to "brush it off" but cannot, finally leaving Michael several years later. When she does leave and Carmela dies shortly thereafter, Connie comes around and not only forgives but also praises Michael, and becomes the main female consort of the family - something Kay was supposed to be but in the films never could. It stands to reason that in this role in the films she would continue to execute the dutiful resignation to terrible but necessary deeds and open secrets that Kay had executed in the novel.
