This is a stupid blog post. There are two goals:

1) Prevent copies from being leaked 2) Trace leaks if they happen

Watermarks are actually great for the first goal. They are big, ugly, and obvious. This discourages people from leaking. If they do leak it's crystal clear they're taking a huge risk. This is also why the DVD screener is low-res SD.

HBO already tracked down where the leak came from. Whatever methods they have in place to do so were successful. Thus any complains about watermarks being ineffective at goal #2 are irrelevant.