yes, landfills are air-tight, and air is required for decomposition to occur. I can't remember if I saw it on the documentary "Garbage Mountain" (but I've seen it referred in several places), but there are sociologists who have bored into landfills to see what's what, and there are completely intact 50+ year old newspapers and food.
Most biodegradable product marketing is just to make people feel good, IMO. *However* something that factors into what I purchase is that trash doesn't always end up in a traditional landfill!! Things get blown out of trash trucks, etc. Or they get towed around on barges .... ending up in the sea ... I *do* feel better buying biodegradable products, even though I know they won't really degrade until someone does something different than traditional landfills. I do it also to encourage that industry to become more profitable.
There is a ton of info on the 'net that will answer that question better than me, but specifically with regards to disposable diapers, the short answer is NO they do not, will not decompose in a landfill. What you didn't ask is: what's the alternative? And, in my opinion, the only plausible solution to this problem is to create less waste, not just pretend it's "better" because it's biodegradable. That's why we don't use disposable products in our home. Unfortunately, people like us are in the minority, and that's why the landfill industry is booming!