What does that mean, "passively charged battery pack"? Did we invent batteries that slowly regain charge over time and I somehow missed it?

I imagine it means a normal hybrid, in contrast to plug-in hybrids.

Normal hybrid will at least regain charge when you brake (and maybe from the gas engine?). You don't actively recharge it.

Passively charged in the sense that the driver cannot actively force it to be charged, either while driving or by plugging it in (it cannot be plugged in).

Vehicles with a stated electric-only range are typically PHEVs, which can be plugged in. Some non plug-in hybrids can be driven in electric-only mode for very short distances, at low speeds only.