absolute BS. by pulling water out of your muscles and bloodstream, you have a higher concentration of T levels. combine that post-workout with the T released from a heavy squat/dead day, and your muscles have become saturated with T, which makes it easier for the T to bind to a receptor.
this is not a recommendation for dehydration, simply 15 minutes in the sauna is beneficial to a small degree. for recovery, the steam room does me wonders. the humidity helps pull blood and nutrients into my muscles, and recovery is much quicker - at least for me.
as far as gains being reduced from a sauna, that is generally spoken by people who forget that they need to replace the excess lost water by drinking more. hydration out of the sauna is a severely overlooked aspect for some.