We can say that it is the first post I mean the first real post. If we include the post that is used to test the forum then thw post made by satoshi nakamoto is not the first one but when it comes to serious discussion then that post is the first one. This is the very first account created on bitcointalk forum with a 7 posts called
admin. Many people didn't consider one of the 7 posts as the very first post as the 7 posts are just for testing if the forum work and runs perfectly.
Ah, you don't need to complicate every things and argue what's there and what's not, in every website there's always a test. A public release/post will be always considered as the actual, not the hidden one. People don't talk about the deleted/hidden post/threads because probably it's unnecessary for public that's why its hidden/trashed.