I don't think there is any problem with replying to / posting on an old thread. He was pointing out (to me) than my saying "I can't wait to see your first project" was probably a bit out of date, given his first project was probably 2 years ago. Nothing but some good-natured ribbing.
If I had asked a question about the thread topic or contributed my experience to the the topic of the thread, it would have not been commented on at all, and would likely have kicked off another round of discussion. This is a good group and very friendly.