Okay. So explain what he was thinking.
We were discussing the case about the teacher and he thought it relevant to bring up that verse. My response was to point out how we sometimes do NOT pray in the privacy of our room and I then gave examples of how we do so without hypocrisy or showiness.
Then you asked: what IS the problem?
So I explained that using that verse to say the teacher was wrong to bow her head in public would make quite a lot of christians who might, say, quickly and quietly bow their head in a restaurant before a meal, or in response to someones request for prayer with them, or in the midst of a sudden disaster when one cannot go running to a private place but needs God's ear NOW, to be called sinners because of the verse.
So then, explain what he meant if you think you know.
I suppose he might have brought the verse up for a completely different reason than the case we were discussing and I just am not aware of it.