I don't think that is true.  It's one of the problems with calling it "covert AsicBoost". It gives the false impression that it is "wrong" or "bad" or "secretive".

The entire point of AsicBoost is to generate SHA256 hashes as efficiently as possible. The point of "covert" instead of "overt" is that with "covert" you don't need to make a mess of the block version number (which is better used for actually keeping track of the block version).  The fact that it is not easy to tell that a block was mined using the "covert" method is a side effect, and not the point.

Ignoring AsicBoost for a moment, you'll notice that it is impossible to tell if any given block was mined with an ASIC (instead of a GPU).  People use ASIC because they are more efficient.  Nobody calls them "Covert ASICs". The point of using an ASIC isn't to hide the fact that you are using it, that's just a side effect.  The point (just like with AsicBoost) is to mine as efficiently as possible.