I don't think you're overreacting at all OP. I watched the video you linked, had a look at the comments, and checked out the rest of her channel.
ASMR has become very mainstream lately, but wasn't always. The first time I heard about ASMR approx 25 yrs ago was through a boyfriend and guy friends. It was known for being a particular kink, not always sexual but definitely erotic. You find tame "background" noise type ASMR easily now, but it's always been around as this sensual kink. For me this video is a classic example of it. She's simulating that whispery intimacy you have with a partner before/after sex. Most of the time the camera is focused on her mouth. She is acting flirtatious, and saying suggestive phrases. Anyone saying this video isn't sexual might not find it floats their particular boat, but that doesn't mean that isn't the intention.
I would also argue this can be used as an intimacy substitute. A lot of men only experience intimacy through sex/sexuality. So many threads on Mumsnet from women whose husbands say sex is how they feel close to their wife, but they are emotionally shut off in the relationship and no intimacy otherwise. The majority of the comments seem to be men, some even saying things like "Day before Valentines and here we are", calling her a "good girl" and referring to her in a weirdly overfamiliar way. Parasocial relationships.
On her channel, video titles are suggestive and some mimic porn genres. "I'll put you to sleep..." with a pic of her lying pouting in bed in a white lace nightgown. There is one of her dressed up as a barmaid with clevage. "ASMR twins". "ASMR in bed with you", "You are in my bed", "Valentine's Day with your goth girlfriend". "Big sis takes care of you" (super gross).
There's been a lot of very nasty responses. OP I do think this is triggering for you because of something in your past, but I don't think that makes you the issue. It's telling you something important about what you need in your relationship: honesty and trust. It might seem small to some people, but he hasn't been honest with you. He framed it as past not present. I'd argue if it really wasn't a big deal, why did he mislead you. Maybe it is a kink for him, and is embarrassed. That's ok, even if it's not something you agree with. But what you need is him to be honest and building trust with you. He didn't do that.
Small things become big things, or represent big things. You have had a strong reaction, and honestly I would have been extremely upset as well. This would be a deal breaker for me, because I wouldn't want to date someone who listens to this type of sexualised content. That's something you need to reflect on for you - is this a deal breaker? Did his explanation feel genuine, or did you still feel uncomfortable for some reason?
Whatever it is, please choose yourself. Don't shrink to tolerate it because you think you have to. If you do it, do it because you want to, and feel at peace with that.