That’s correct! When TBC Classic launched, there was a snapshot taken and players could decide to go to TBC Classic or Classic Era servers (and pay for a clone for the other). However, and characters created after that point in time on either Classic Era or TBC Classic do not have that same opportunity.
When Wrath Classic launches, as far as I know, TBC Classic servers and characters will upgrade to Wrath Classic servers and characters.
So, yes if you want to play on a Wrath Classic server, you can either roll a character on a current TBC Classic server, or wait for the new fresh start realms:
There are guilds getting ready specifically for Fresh WotLK Classic