That's a case of someone who thinks they understand what they're talking about.
Let me be perfectly vague about this: Putting your focal point in the middle of the canvas is not bad all the time, but not good more times than it is good.
Now let me clarify what I meant: A lot of people say "don't put the focal in the middle" because they've learned the
Rule of Thirds. The problem is, after learning about this, they take it as absolute fact and try to act smart by telling everyone else that they know what the "right thing" to do is.
The Rule of Thirds is a good thing to know. And a lot of the time, having the focal off-center actually HELPS the flow in the tag. But there are more than enough reasons that you, the artist, can use to justify putting the focal in the center of the tag. Sometimes, the tag will actually look better that way!
...I wrote too much again, didn't I?