Part of the problem is that we actually know very little about the workings of the Potterverse.
For example, we all know that the Elder Wand is a big deal. There’s a whole book about the Deathly Hallows. But what does the wand actually do? Other than Weasly’s malfunctioning wand, I don’t think the book ever discusses the role the wand plays in magical duels. They make a big deal about matching a wand and a wizard, too, but without ever making it clear why that matters.
There’s also a lot in there about prophecy and foretelling, but other than extremely rare prophecies (like about Harry), all we see is a silly and likely wrong professor reading tea leaves. Surely there’s more to it than what we see in the books. So maybe it’s feasible for someone like Voldemort to see an attack coming.
And dueling instruction? Most of what the kids learned was reluctantly taught by Snape, eagerly taught by the incompetent Lockhart or figured out through their own experimentation. I refuse to believe Harry knows the first thing about dueling, no more than I know about football because I watch the Superbowl.
So, anyway, there are two ways to go with this if you want to analyze it. Either
- go the HPMOR route and assume that wizards are idiots and Harry is really smart or
- ask yourself what an 11 year old abused and isolated child knows about anything, and why we should take his word for how the world works. Even by the last book, he’s only 18 and any of us over 30 can look back and realize how little we knew about anything at 18.