This is not true!
When working with pet moving companies like ours (PetRelocation) we work with airlines that only have pressurized & climate controlled cargo holds.
In fact, an aircraft is somewhat like a balloon. It is impossible to inflate part of a balloon and not the rest, right? So how do you suppose they would pressurize part of the aircraft and not the rest? They can't. It defies the laws of physics. The air in the cargo holds and in the passenger cabin is the same. In fact, an airline will not accept a pet that has fouled its carrier for just that reason -- they don't want passengers complaining about the odor.
The air is circulated throughout the entire plane - so most airlines only allow a minimum number of pets on board, as the potential for their smell to carry through the cabin is very possible.
All of these cargo holds are insulated, climate controlled, pressurized and they even have slightly dimmed lights. This allows the animal to travel in comfort, just as we do sitting above in the cabin.
If you think about it from the pet's perspective, they do not know they are flying in a plane. To them, they are being placed into a special 'room' that may shake a bit on take off and landing - but other than that, the pets are simply waiting for that individual who put them there, to come and open that door so they can enjoy their new location. Little do they know, that its not the same location they left from! :-)
Most, or almost all, of the stress a pet will experience when flying - is the waiting period on the tarmac before/after its flight. Since pets are last to be loaded & first to be removed, many airlines will send all the 'cargo' to the plane at once. As the pet waits its turn to be loaded/unloaded, it must sit on the tarmac until its their turn.
It is this wait, that can cause the most stress to a pet. When working with pet shipping companies like ours, PetRelocation, we work with the airlines on loading/unloading the pets separately from the rest of the cargo. This basically means that the pets will never sit on the tarmac, as the airlines will shuttle them to/from the plane when its time for them to load/unload - rather than sending them with the general cargo.
All in all, it makes for one stress free journey for the pet!