No one really answered your question, so I will send a post. The reason they say not to use that heater above a certain altitude is the lower oxygen level. When you get to a certain level you can start producing CO, which is deadly. Also, use of any kind of fuel/air heater in an enclosed and unvented space is foolish, catalytic or otherwise.
The same thing applies to forge burners. If you go to a high enough elevation you will need to reduce the gas orifice diameter, or it will burn rich and produce CO. The burners on my web site, the Reil and EZ burners, and the Rex linked burners, can all operate well up to at least 7000 feet, although you do need to pay attention to your fuel air mix. I know that because smiths send me e-mails about easily forge welding with the Reil or EZ burners at elevations as high as 7000 feet. No one has ever sent me an e-mail about elevations higher than 7000'. It is a good idea when using the Reil or EZ burners to use at least a 2" diameter bell fitting, and the use of a TWECO tip is recommended too. Look carefully at the fuel/air mixture flame images I have posted at
and be certain you can achieve a neutral burn. If it burns rich, then you are producing CO, and that can kill you if you are in an enclosed space. Then you need to reduce the jet diameter.....before it kills you!