AddChatContainerTab(containerId, name, isCombatLog) is just one of the functions you have to call to make new chat window (tab) working. This will only create settings for the window, but it does not update chat system.
CHAT_SYSTEM itself uses CreateNewChatTab method, but it will also show options for the new window, so you might want to use just chatContainer:AddWindow, see examples below:
Lua Code:
local container = CHAT_SYSTEM.primaryContainer
--this command creates a new chat window and displays tab options:
CHAT_SYSTEM:CreateNewChatTab(container)
--this command creates a new chat window but does not display options:
local name = "New Tab"
container:AddWindow(name)