At 8 layers of management (so 9 layers total, with the bottom rung being non-management), 3 reports per manager comes out to 6561 employers on the bottom rung. At 5 reports each, that 8 layers would give you over 300k at the bottom, an 10 each would give you 100m at the bottom.
> We’re flattening our organization because eight layers is too deep
Eight layers total
So... That's the assumption that would give you 300k employees with 5 reports per manager. Sum of 5^x for x in [0, 7] is ~300k.
The GP miscalculated it.