It turned out that Linus had confused "IBRS" and "IBPB". That seems to me that he didn't understand it.
I'm not sure why saying "I don't understand this" would confer _incompetence_ rather than just not understanding. Understanding is easily gained with good communication. The original patch set clearly doesn't explain the point of the patches well, and everyone gets things wrong as well. There's no shame in saying "I don't understand this", it's not a final statement equivalent to "I will never be able to understand this".
I'm not sure why saying "I don't understand this" would confer _incompetence_ rather than just not understanding. Understanding is easily gained with good communication. The original patch set clearly doesn't explain the point of the patches well, and everyone gets things wrong as well. There's no shame in saying "I don't understand this", it's not a final statement equivalent to "I will never be able to understand this".