I think the key point in that saying is that you don't want to let what they say or do control you too much. You don't want to get discouraged over one comment or action.
Not believing what they say.... I think they even believe it, when they say it. So, from that point of view, it is true. But, at the same time, they are often very confused. So what is true today might not be true tomorrow.
On top of that, if there is an OP involved, it's almost certain that there is out and out lying going on besides.
As far as believing their actions, I think the key is not to place much stock in individual actions. Rather, you are watching for patterns over the long haul. And that's also how you start to get a feeling for what's working.
How are you supposed to know what's going on with them? From all the situations I've read here, you just can't do it. Tea leaves might help, I guess. Since you can't know what's going on with them, you need to stop focusing on them, and really move your focus to yourself.
