Posted here because I'm 99% sure this mod was what allowed this to happen. So, in Lockdown DLC, you would hear L4D1 survivors' voices throughout the campaign, and I guess that the map creator needs to physically summon them to play the voice lines. Well, apparently, if you use bots warp after these voice lines play, the survivors that spoke well teleport to you, and since I saw Francis with a melee weapon, they are definitely using Left4Bots settings.
I discovered this in chapter 2 when I saw Francis, Louis, and Zoey on the roof of some building for the first time after exiting the first underground area, and curiosity took over. This map was in my collection since my playtime was still under 100 hours, so it was quite a surprise. After teleporting them, I thought they didn't have a proper AI since they just sort of stood there with their pistols, and I couldn't give them a command, so I carried on. However, later on, when I was in the building where you have to turn on the generator for the door, on the top right of my screen was "Louis killed a Boomer," and then appeared Francis, Louis, and Zoey with some gears. They followed me until the chapter ended, and they disappeared when the next chapter was loaded. Then, I just repeated this trick every time I heard their voices for each chapter.
Interestingly, it seems that they follow their own direction like a separate team, unlike any other experience I have with "extra survivors" mod or map out there. It has always been my headcanon that the L4D1 survivors were just ahead of the L4D2 survivors in this campaign, so this is pretty cool for me.
However, it seems that this is only really helpful in chapter 2. In the other chapter, they disappeared after the immediate event that they were in.

Chapter 1: Bill is available at the stairs, best to warp him before the horde start.
Chapter 2: Francis, Louis, and Zoey, after exiting the underground area, will help you all the way to the end of the chapter.
Chapter 3: Louis appeared in the area with the scripted tank fight and disappeared after you go down.
Chapter 4: Zoey helps you turn on the generator and gone before the double tanks start.
Chapter 5: None.