Spin glass behavior is traditionally attributed to quenched disorder and frustrated local interactions. However, recent experiments in multimode cavity quantum electrodynamics (QED) systems have demonstrated the emergence of replica symmetry breaking and ultrametric organization in the absence of conventional microscopic disorder. This work analyzes experimental results reported in a driven–dissipative multimode cavity QED platform, in which atomic spins interact through a small number of global photonic modes. Despite the lack of random local couplings, the system exhibits hallmark features of spin glass physics, including hierarchical state organization and ultrametric overlap structure. We show that these observations admit a natural physical interpretation in terms of global mode geometry. In this framework, atomic spins act as boundary conditions on a constrained electromagnetic field, and ultrametricity arises from nested compatibility conditions between collective modes rather than from combinatorial frustration. Replica symmetry breaking emerges as a statistical signature of this underlying mode geometry, not as a fundamental property of local spin interactions. This interpretation is complementary to effective Ising Hamiltonian and replica-theoretic descriptions, which successfully capture the statistical phenomenology. By identifying a physical mechanism rooted in global mode constraints, the present analysis provides insight into how glassy order can arise in driven quantum systems without quenched disorder. Together with independent results in gravitational and quantum foundational contexts, these findings support a geometry-first perspective on collective ordering phenomena and place strong constraints on disorder-based interpretations of spin glass behavior.