That's true on models which have dedicated stack ports. For models that have flexible stacking, technically a valid optics configuration can work as well.
Aside from this caveat, physical stacking provides a better redundancy and more flexibility than a warm spare (VRRP) configuration. In addition to opening the door to cross-stack LAGs (which can eliminate STP blocked links), a switch stack can also use dynamic routing and multicast routing, which is not supported in a warm spare configuration.