"It" is an indefinite pronoun, meaning, "it" is by definition unclear to anyone but the speaker.
Here, "it" means "your involvement in this discussion"
To put it a bit more plainly, and apologies if it is hard to hear currently, I'd find this very helpful to hear this if I were you:
- The first comment was objectively, straightforwardly, dismissive. You questioned one statistic, then ignored the thrust of the comment and the comments leading up to it, instead framing the difficulty as "modern computers have a hard time with 3D", eliding the main point communicated.
- You then replied to my comment to tell me I'm wrong, it wasn't dismissive. This is also, in itself, dismissive. I had already fisked you, and I felt bad, so I didn't want to get overly literal and criticize you roughly again, in front of a crowd, so soon. So, I choose to keep it brief, rely on your ability to recognize the irony in your reply being dismissive in telling me the original comment wasn't dismissive, especially when combined with the wink.
To be hyper clear, it's...unusual...when communicating with others to ignore most of what they just said and fixate on one illustrative part of what they said, then when told you were being dismissive, to just say "I wasn't being dismissive" and put the burden on the listener. It's so unusual as to be objectively amusing.
You certainly did not mean to be dismissive, but you were. I sympathize with the reflex, I am 100% sure I've made many similar errors in my own journey, probably more than you ever have and will. It is a problem area for me.
With humility, I'd humbly suggest that when given feedback, rather than briefly asserting the feedback is incorrect, you come with curiosity and ask why it came off that way.
Given their answer, you'll be able to tell very quickly if the person is having a bad day/picking on you, or if you were unclear.