  Eigenstate thermalization hypothesis (ETH) explains how generic observables
of individual isolated quantum systems in pure states can exhibit thermal
behaviors. ETH ansatz usually holds true in quantum chaotic systems. In this
paper, we examine a one-dimensional lattice generalization of the
Sachdev-Ye-Kitaev model with spatial local random interaction of Majorana
fermions, the so-called SYK chain model. The model is maximally chaotic but its
R\'enyi entanglement entropy study suggests that the model does not rapidly
thermalize. We show, however, that for two conventional few-body operators, the
ensemble-averaged theory of the SYK chain model strictly satisfies ETH
conditions. We also demonstrate that for every single realization of the
ensemble, the operators loosely satisfy ETH, and consequently rapidly
thermalize, albeit, with larger fluctuations. We comment on the difference
between the application of ETH for individual systems and ensemble-averaged
ones. We also use our results to comment on the implications for eigenstate
correlation functions and a putative gravitational dual theory.
