I think you’re missing two points:
One, it’s a pressure vessel that contains high pressure. Even the tiniest explosion that compromises the pressure vessel can then use the energy in the compressed gas to complete the explosion. We’re not talking fragmentary hand grenade, but if you’re in the path of that tank when it goes flying across the room, the difference won’t matter to you.
Two, that tiny whisper of hydrocarbon might just be the start of the fire. Think of it as the fuse, or as kindling. Once a fire in easily-flammable hydrocarbons start in the presence of pure oxygen, It can very quickly move onto the next source of fuel. And like I said, with oxygen, things you don’t normally think of as fuel are. Like the aluminum walls of the pressure vessel. Or the carbon steel body of the valve. Or the hydrocarbon rubber O-rings. Or the rubber hose you attached to the tank.
And the final thing is this: I’m not trying to make it sound like there’s a high degree of likelihood that any of this will happen. As was mentioned in the Oxy Hacker’s Companion: if these events were likely, then a lot of welding shops would be blowing up all the time. But the fact is, these type of events can happen, and even if it only happens one in 1000 times, if it’s something you do with any kind of regularity, it will happen to you.
Hence the warning. With just a tiny bit of foresight and proper procedure, you can eliminate the danger.
And to answer your ending question: while dealing with compressed *air*, you don’t have to worry about fire. Remember, in warning number four, it specifically mentions a container of oxygen. In this case, it’s not using “oxygen“ in place of air like some ignorant reporter who says all of us scuba divers breathe “oxygen tanks“. They’re specifically talking about a cylinder of oxygen as well as a cylinder of air. If you don’t have pure oxygen, you don’t have the same circumstances, and the warning does not apply.
The question comes when someone hands you a cylinder to be filled. Do you know what’s inside of it? They might tell you what they think is inside of it, but numerous scuba divers have died because they swore up and down they knew what was inside their tank, but they were wrong. So without checking, you have no way of knowing what is inside that cylinder. And therefore, warning number four theoretically could apply.
That’s why, especially among technical divers, the rule is: every cylinder is analyzed every time and labeled right before you use it. (Generally, that same day.) Because we *do* have the possibility of pure oxygen cylinders laying around, and sometimes all those scuba cylinders can look exactly alike. And there are numerous ways that not knowing what is in your cylinder can kill you. Warning number four is just one of them!
ETA: And to emphasize, this is not necessarily a significant risk. It is a *serious* one, but not necessarily a significant one. Which is why you probably haven’t seen that warning before, and why it’s not splashed all over the place. It doesn’t make the warning any less accurate. It also might mean that the circumstances that led to that warning in that manual might be quite specific to that application, and can’t simply be extrapolated to any and all activity involving unknown compressed gas. Like, if the task was using a high volume and high pressure compressor to quickly fill containers, then yes, it would be smart to warn people that not all containers can be treated the same! Just because you can do it with a vessel partially filled with air does not mean you can do it with a vessel partially filled with oxygen — and that’s especially relevant because so many people think that air and oxygen are the same thing!
