That's not technically a "Mains" panel.
Your electrician may have to get a little creative, or upgrade to something that'll work. Try to find an older electrician named "Butch" or "Sparky", not one with a Millennial name (e.g. Jared or Conner). The older guy has probably seen everything and will figure it out.
Edit: Not sure if to code, but...if the electrician installed the generator breaker opposite the service disconnect breaker, then joined the handles with pins, you'd be able to run only one breaker at a time, sooo...ask your electrician if that's legit.
Or something like this, but installed in reverse:
https://www.garvinindustries.com/panel- ... cEQAvD_BwE
Re-edit: Forget about the pins...that might prevent the service disconnect from tripping when needed, but the above device (installed in reverse) would work with some tweaking.
GE does make some oddities in the lock-out department, though...you might get lucky with a little research.

- I can explain it to you, but I can't understand it for you... -