This isn’t quite right. The trilogy isn’t perfect but it is mostly consistent. Marty and Doc are both time travelers in a different t time than they should be in the second. As such they were, in fact, not aware Biff had made any changes until they traveled back to a divergent 1985 already affected by his change.

As time travelers, they retained memories of the changes just as Marty upon returning to 1985 at the end of the first did. He was the only that could know things were different. Even Doc could not know. The author is pointing out that everyone Marty met in 1955 should in 1985. Having no knowledge of the existence of time travel, his parents would likely believe it a coincidence that their Son Marty reminded them so much of that kid they knew briefly for a week 30 years ago (that they actually named their son after). They credit Biff with bringing them together instead of that other kid. Doc brown, knowing about the Time Machine would have had a clearer memory of the events 30 years prior.

The films present a timeline capable of changing but never more than one simultaneous timeline. The chalk drawing in the second is just Docs attempt to illustrate for Marty the reasoning behind not being able to correct a change in 1985 that was the result of a change introduced in 1955.