  The glassy thermal conductivities observed in crystalline inorganic
perovskites such as Cs$_3$Bi$_2$I$_6$Cl$_3$ is perplexing and lacking
theoretical explanations. Here, we first experimentally measure such its
thermal transport behavior from 20~K to 300~K, after synthesizing
Cs$_3$Bi$_2$I$_6$Cl$_3$ single crystals. Using path-integral molecular dynamics
simulations driven by machine learning potentials, we reveal that
Cs$_3$Bi$_2$I$_6$Cl$_3$ has large lattice distortions at low temperatures,
which may be related to the large atomic size mismatch. Employing the
regularized Wigner thermal transport theory, we reproduce the experimental
thermal conductivities based on lattice-distorted structures. This study thus
provides a framework for predicting and understanding glassy thermal transport
in materials with strong lattice disorder.
