There's a large difference between thoughts about suicide, and intentions to act on it.

It sounds like your T is accepting your feelings and demonstrating trust in you; which sound like things you don't have in yourself. It's ok to be confused by their reaction and totally fine to ask about it when you see them next.

Your T is likely paying a lot closer attention to you right now, and will do so for the next while even if they don't say anything, they're probably trying to show you that you can trust them. If a T was going to overreact and freak out, it would make the client feel ashamed, angry, or decide to never disclose again. Which wouldn't be helpful.

A lot of the time, opening up and talking about the suicidal thoughts can be enough to help curb them.

I'm sorry that you feel like you weren't being taken seriously. Sometimes it's just a shock to our systems when someone accepts us and doesn't react in a more cruel/unsensitive manner (which is what a lot of people who deal with suicide are used to).

My T even knows my plan - it's not an easy one to act out so it sort of works as a prevention plan for me. He's made jokes about my typical horrid coping skill (overdosing on over-the-counter-meds). I don't feel invalidated at all by those; I actually feel more validated. He's heard what I've said and he apparently trusts me enough to not make any stupid choices even when he knows I might want to. He knows that death doesn't worry me, he knows a lot of things actually that could sound like I'm in serious trouble. All of which I said within one session I think. He told me what part of my conversation concerned him the most - that I can talk about it rather matter-of-factly.