I read the co-founder's comment as the whole company having 1000 employees (Wikipedia confirms this) and the vast majority serving the core functionality in one way or the other.
Without Sales, Ops, etc. I imagine the number of core functionality devs to be 100-300.
They have a calculator on their website [0] to compare your wealth ranking in the world with and without donating 10%. I believe for many people in IT it barely makes a difference (whereas for the recipients it's oftentimes life-changing).
It does not neccessarily imply mastery in all layers of the stack but rather familiarity and often the skills to understand and fix bugs in other areas other than your field of expertise.
Without Sales, Ops, etc. I imagine the number of core functionality devs to be 100-300.