In 1975, Buchwalter and Closs reported one of the first examples of heavy-atom quantum mechanical tunneling (QMT) by studying the ring closure of triplet cyclopentane-1,3-diyl to singlet bicyclo[2.1.0]pentane in cryogenic glasses. Since then, no clear theoretical evidence has been provided to elucidate how the intersystem crossing (ISC) and QMT are related in the reaction mechanism. In this work, we unequivocally show that at cryogenic temperatures, the ISC occurs solely in the quantum tunneling regime, with weak coupling non-adiabatic transition state theory rate constants predicting a spontaneous reaction in fair agreement with experimental observations. Despite its limitations, such an approach can be used to help unlock a comprehensive understanding of a variety of spin-forbidden chemical reactions in the low temperature regime.