It's a combination of t levels going up and the false assumption you made in your mind that it's curing the stutter which created a placebo effect that allowed you to relaxe with an elevated sense of confidence. I lift weights regularly and yes I get into high T states where I feel like I can punch space and time, but if it's not congruent with my mental states it won't make me fluent. If you had to do a high pressure phone call or negotiation the stutter would manifest.  Socializing in a zone, like being drunk and talking fluently with friends, is low pressure communication.  Fluency is being able to speak even under duress