Actually, I have found that the information given in the article is spot on. I have used multiple sets of tires from multiple brands on multiple sets of wheels on multiple vehicles, and all have lost 1/2 to 1 psi of pressure per month. However, the difference is not usually noticeable because, as I noted above, the difference due to air loss through permeation is not as significant as the difference due to temperature changes.
For example, when I go from March to July (four months), typical outside daytime temperatures here go from around 40F to around 80F. Without adding or releasing any air, that will cause the pressure in the tires on the car to go up by 4 psi (rough estimate, again per the Tire Rack). Loss of air due to permeation will cause the pressure to go down by 2-4 psi over the same period. So overall, the pressure isn't going down at all; you may erroneously think that there is no permeation, but what you're actually seeing is air loss due to permeation being offset by pressure increase due to temperature change.
In the fall, going from July to November, temperature goes from 80F back down to around 40F, so the pressure goes down by about 4 psi due to the temperature difference, plus another 2-4 psi due to permeation. So the tires on the car lose 6-8 psi, and you need to add air to them to compensate. You may think it's only due to the difference in temperature, but there's a permeation difference on top of that. Of course, this is happening over the course of four months, and with frequent checking, I'm usually only adding 1 psi at a time; if I didn't know better, I might not realize how much air I'm adding to the tires over a period of time. Those whose climate is pretty moderate year round, where temperatures don't vary all that much, may not realize that they're adding 1-2 psi once every couple of months. Or may not have an accurate tire pressure gauge. Or may not be checking their tires at all.
It's also possible that someone else is putting air in your tires and you don't know about it. I know a lot of dealers/mechanics check your tires when you go for service, even if it's just an oil change. I know this because I've checked tires when the car is back from service, noticed differences, and asked about it.
As you can see, the difference due to temperature is greater than the difference due to permeation, so the latter isn't readily apparent. The reason I know that permeation takes place and that 2-4 psi difference is accurate is because I'm switching between summer and winter tires at roughly the same temperature each year, and I always check the pressure when I put the tires back on the car. The summer tires typically lose 2-3 psi during the 3-4 months that they are off the car, and the winter tires typically lose 4-6 psi during the 8-9 months that they are off the car.
Tires should not be loosing that much air in that short of a period.
I would bet that if you kept precise track of the pressure in your tires, and how much air you were adding to them, over an extended period of time, and kept track of adjustments due to temperature, you would find that your tires are losing the same 1/2 to 1 psi per month aside from temperature changes. If you don't believe this, try it - not just for a month, but over the course of a year or more. Use an accurate tire gauge, check your pressures cold (when the car has been sitting out of the sun at the temperature at which it will be used), and keep track of the pressures and temperatures, and how much air you (or anyone else) add or release. You'll see it for yourself.