I don't think people are missing his point. It is just that his point is not inline with the discussion. Walls, as is being described in the picture and by other posters, only have one purpose and that is to be a barrier to something. So the statement that

Obviously the wall in this picture and the "Great Wall of Trump-merica" serve vastly different purposes.

is not an accurate representation of the picture as the topic is not specific to a wall as a physical thing you could touch but rather to the idea of what a wall is.

To make the quoted comment make sense the picture would have to be specifically talking about building the DNC wall as its physical form at the border.

To try to put it another way, the picture is not trying to say "oh you built a DNC wall at your convention but refuse to build a DNC wall at the border." It is saying, oh you built a wall, as an idea, at your convention but refuse to build a wall, as an idea, at the border.