The OP is a little confusing. The first two weeks of January are often booked up at 7 months advance, and certainly looking now I would expect availability to be very low 4 months in advance. This is nothing new, it has been going on for years as a lot of school holidays extend into early January, then you have the marathon and then you have MLK weekend, which is also very popular.
Once you get past MLK weekend, there is a bit of a dead period where there is usually a lot of availability between the last two weeks of January and the first two weeks of February. However, 4 months in advance I would expect even this to be pretty booked in studios. Then you get into school breaks, and popularity increases, but even so there are usually some availability at 7 months.
To the other part of the question. No, Disney is NOT overselling
DVC. They legally cannot do that. But what happens is as more and more DVC resorts are sold off, the most popular ones are bound to become harder to get at 7 months. For instance, all those people buying at Riviera may love their resort, but when they realize they can stay at Boardwalk or BLT for less points, they are going to want to try that. And regardless of where people own, they all want to try Beach Club at some point. That resort has been extremely hard to get a studio at for years now, but only has gotten harder across all months. However, I don't believe Disney is playing any games here. It's natural demand.