Reading is thinking someone else's thoughts => That is true if you are strictly reading passively. Typically what happens is that reading opens many doors that leads to your own thinking. Of course depends on the type of material you are reading as well. But often reading broadens your thinking relative to just putting your own on paper.
Definitely a good point. I live in a college town and know many people that read all the time, but don't actually do anything active with what they've read. They just consume it continuously and think the understand many topics. Except when you talk to them, it comes out quickly that they didn't actually understand what they read on a deep level, they just went along for the "thinking ride".
And, as you point out, if you push yourself to read actively, it helps a lot!
Writing is thinking your own thoughts.
There's a big difference, and is why writing is so painful for so many people. It's also why writing is critically important.
edit: Likewise teaching is really important. Crystallization of thought is incredible valuable and difficult.