http://www.urbandictionary.com/define.php?term=Sandwich%20Ar...

    2. A person who, in real terms, is unskilled, and cant 
       see it, despite thinking they have a 'talent'. The 
       person often makes inappropriate, obtuse, or 
       simplistic comments, to everyone elses amusement. 

So you're saying he's really insulting the people who came up with the phrase "Sandwich Artist", not the people who hold it as a job title? Because that's ridiculous. He used the phrase "Sandwich Artist" to evoke the image of an unskilled person who thinks they're much more important than they think they are. It comes from the fact that Subway employees are called "Sandwich Artists", despite their job being much the same as any other fast food server.

It does not take a degree in linguistics to understand that using someone else's job title in a derogatory manner is shitting on the person with the job title. This is something you should have learned in grade school.