I'm not going to get into whether he is right or wrong about this (you've gotten lots of feedback on the end), but instead, challenge you to challenge your own thinking about SH a bit since you also mentioned you realize you may not be thinking clearly about all of this.

You refer to SH as
"my biggest, best coping mechanism"
"that one thing I planned on"
"the one coping mechanism that always works"

Yet,
Your T offered to help you with some visualization techniques.
He offered to talk to you and debrief.
You did use your social supports.
You texted friends.
You called friends.
They provided support.
One friend offered to go with you.

You clearly have a "toolbox" of coping strategies that are much healthier and much less dangerous that SH; you've been using them. But you seem determined the elevate SH to the "biggest" "best" "one". Maybe it's just the "easiest" one? It's your default. You know you have other, healthier, and probably quite effective coping strategies, but because your default is more automatic for you, it feels like the "best" when perhaps it's just "the easiest".

Right or wrong, your T seems to know you have the ability to utilize healthy coping strategies if you put your mind to it. But I think you are onto the whole "I'm going to buck the authority figure" defiance thing that you seem to have going on here.

Maybe rather than choosing to focus on what your T may have done wrong, reframe this as a time to be as determined and confident as he is that you can get through this without resorting to SH because, as you and he both know, you do have healthier coping skills at your disposal if you choose to use them.

I don't think getting hung up right now on his rightness or wrongness serves you in any way except to perhaps justify to yourself a reason to SH ("He screwed up, so I'm justified in SH.") You can talk to him about that later, but don't use it as a rationale for resorting to what really is your least safe, least effective, least healthy coping mechanism.