The Short Version
The evidence supports the claim in broad terms. Heat or thermal stress can be associated with higher cortisol and lower testosterone, and glucocorticoids are well documented to suppress testosterone production. The main caveat is that heat can also lower testosterone through other pathways, so cortisol is a plausible mechanism, not always the sole or primary one; effects on sexually dimorphic traits are better supported for sustained hormone changes than for short-term stress.