This isn't true.[0] The difference between "Ich bin Berliner" and "Ich bin ein Berliner" is that the former says that you literally are from/living in Berlin, and the latter means you are figuratively speaking as like a Berliner. "Ich bin Clown" means that your job is to entertain people with face-paint on, whereas "Ich bin ein Clown" means that you sometimes behave clownishly.[1]
[0] https://en.wikipedia.org/wiki/Ich_bin_ein_Berliner#%22I_am_a...
[1] https://language.mki.wisc.edu/essays/ich-bin-ein-berliner/