Can someone translate this to explicit terms? My layman (non-MBA) brain thinks this is a blackmail threat, which seems like a very unexpected place to take all this
Nah - He's just joking that if he starts spilling the company secrets behind him getting fired, the board should try to take his shares. The joke of course is that he famously owns zero equity in the company.
That may mean that he is telling that nothing is stopping him to start the to talk.