Thats an astute observation. Whenever you're tricking someone, you put something stupid easy in for the tricked person to defeat, so they assume they're tricking you.
But it could be even easier. Why not put the watermark in a different location for every copy you distribute?
The silly thing is that nobody seems to realize that the public has only seen 1 leaked copy, probably from 1 source. HBO could be using any number of techniques and none of us would notice any of them because we have a sample size of 1. Even the odd techniques OP describes could be present in the copy, your described technique as well, any number really. We won't see them unless they are as in-your-face as the watermark.
But it could be even easier. Why not put the watermark in a different location for every copy you distribute?