Yes, that's true. (And the Senate is no different.) What does it mean?

I think it means that people do not realize the actual problem. They don't see the two facts you cite as at odds with each other or connected to each other at all. But they are. The reason why Congress can have such a low approval rating while incumbency reelection rates remain high are that people think it's all those other members of Congress who are the problem--if only everyone would listen to their members of Congress, all that stuff would get fixed. They don't realize that, if you send someone to Congress to fix something, and it doesn't get fixed, you need to send someone else. You can't keep allowing the incumbents to hide behind "it's not me, it's all those others" forever.