It looks like the conclusion is overly simplistic (but it's a blog entry based on a single experiment, so it's quite reasonable to limit it to simple conclusions)

If I understand the data correctly, Sunday has a low open rate, but of those that open, it had very high engagement rate.

The author seems to be saying that the open rate is low enough to make Sunday a "bad day", but that really depends on what your email is about.

If getting people to read the email is your priority, then avoid Sunday, but if you mostly care about getting them to respond in some way, then Sunday could be good - those that read it will have the free time to actually respond.

(Of course, while the limited data provided by this experiment is interesting, it's probably not worth basing any serious decisions on)