Is a “passively cooled” computer the exact same as a “fanless” computer?

Yes, for all practical purposes, the terms are used interchangeably. “Passively cooled” describes the heat dissipation method (conduction and convection via a heat sink), while “fanless” describes the physical design (the absence of a fan). When you see either term, it refers to a rugged, solid-state PC with no moving parts.