Sounds to me like you may have gotten some bad info (misinformation) from the iceberg video. Demo 2 never had the RedXVI Red Volcano secret.
I see. I needed to type Final Deno 1.09.4 .Anyways,lets not argue about one misinformation entire time and focus on thread.
Lets talk a bit more on the SRB2's history:
1. SRB2 TGF was the earliest version of SRB2,however it is in 2D like its prequel,SRB1.It had 3 zones: Greenflower,Techno Hill and Deep Sea,as well as unused Mine Maze.
2. The earliest version of Egg Rock had the music which will be used for Orbital Hangar until 2.2's release.
Additional fact: SSNTails posted a beta version of Egg Rock on Youtube,showing a part where all badniks in the game (at the time) are in some sort of machines.
3.Altrough some of us see the SP zone lineup is complete,it isnt,as there are more zones that are yet to be released in future.Some of them are Doomship Zone and Dark City Zone.
4. SRB2 TGF was originally going to have cutscenes which make transtition from one zone to another.Those cutscenes can be found on SSNTails' channel.
5. Speaking of SSNTails,his youtube channel has lots of things from the past versions,such as SAGE 2000 trailer,SRB2 TGF cutscenes, and there is Sonikku Directs series (which maybe not have connection to SRB2 that much)
6. The SRB2 Christmas demo had an earliest recreation of a Sonic game in SRB2,that being Sonic Adventure.
7. In 2.0(maybe),during the credits,there is a Looney Tunes reference with Eggman on a ACME rocket and Sonic saying "Ding! Ding!"
8. (This one Glaber is for you) Altrough many mods are trying to recreate the past in SRB2,one of the most succesful mods is SRB2 The Past,which will soon get an 2.0 update.The mod features many things from SRB2's past, even including few of SRB2 TGF levels as well as a gallery room.