No, the comparison is right on. Structurally, the two movies are extremely similar: they’re both told from an older/semi-omnipotent perspective of the main character, in flashback, taking key scenes from a long life and showing them to us, then skipping ahead to the next key scene, etc. They even use the same silly visual motifs to make points, like the feather in Forrest Gump and the hummingbird in Benjamin Button.
Story-wise, they’re also extremely similar. I don’t know what bullshit Malcom is spewing about how this story is “standard for any biographical fiction.” What the fuck? Both stories are about a man with some profound mental difference that separates him from the world around him. He’s born early in the 20th century in the southeastern United States. In each case, he has no strong father figure but a very strong, loving, and protective mother figure, whose sage advice – amounting, essentially, to “go with the flow” – he follows throughout his life. In each story he meets the love of his life while they are both children, but in each story they cannot be together at first (or for many years thereafter) because of his handicap, and because the girl is unsettled in herself. In each story the girl goes off to find herself, and so the boy goes off traveling the world having a series of adventures, including such growing-to-a-man pursuits as going to war and learning the sea. During this time, the boy finds a sort of surrogate father-figure, a man who is abrasive and often emotionally closed, but in the end loves our protagonist and helps him grow. In each story, the girl and the boy each pursue their own interests, including romantic interests, but cannot settle on anyone else because we know they’ve been destined for each other since childhood. The boy eventually lucks into substantial wealth. Eventually, in both stories, the boy and the girl burn out their desire to find themselves and realize that what they really have wanted all along is to be together, and so they get together, finally, as adults. In both stories they conceive a single child, but in both stories that child’s paternity is kept secret or in doubt from at least one of the key players, and in both stories the boy and girl can’t live happily ever after, instead becoming separated by fate just after they found a short time of true happiness. But all is not sadness, because in both cases we know that their child, the culmination, end, and beginning of the meaning of their relationship, lives on as a strong and healthy addition to the world.
That paragraph sums up both movies in great detail and leaves out almost nothing of importance. There is not one single sentence in that paragraph that is “standard to all biographical fiction.” Maybe you only watch Eric Roth movies, but there are a bazillion biographical fictional stories that can be and have been told that don’t involve any of those plot points.
Edit: Oh, and the main characters are extremely similar as well. They’re both emotionally flat, almost Zenlike, guys who approach life with a childlike sense of wonder and take the curveballs life throws at them with aplomb. They’re both noble men with essentially no vices at all, who view the benign vices of the men around them with some amusement, but never getting stained by them. They are both fundamentally different from everyone else, but in each case they simply accept that and move on, rather than being bitter or enraged about it. Both go through life with a sense of vague longing and un-fulfilledness, until they hook up with the girl, and then both carry with them a sort of profound shadow of sadness after the breakup.