Occupations and similar words, like political or religious affiliations don’t take indefinite articles in Swedish in this type of construction, just as in German, French or Spanish:
- Han är sjuksköterska. (He’s a nurse)
- Hon är advokat. (She’s a lawyer.)
- Han är rasist. (He’s a racist)
- Han är muslim. (He’s a muslim.)
- Hon är socialist. (She’s a socialist.)
Going by your third example, does it also follow that "han är idiot" rather than "han är en idiot"?
(Also, if there isn't one already, can you please add a module on insults and profanities? It should be one of the first things which people get taught, but it's almost always one of the last, if ever. :( )
Firstly, we’re not allowed have a section on profanities because Duolingo should be open to everyone and be welcoming, including kids and people who are offended by profanities. Secondly, you will pick up on profanities anyway since native speakers usually teach them quite readily.