Axe - That is poor information.
Thatch layers can increase and build for a number of reasons. When thatch, especially in cool season lawns, reaches over a half of one inch thick, it should be reduced. This would be an excellent time to verticut or use a power seeder to first de-thatch and then collect that layer. Next, add the seed to your machine and do your seeding passes. You will still pull up more thatch on these subsequent passes but in most cases, that can be left on op of the grass and it will fall back down to the soil on its own.
I've seen little thatch reduction when core aeration is used for the sole purpose of reducing thatch, although it does help to some degree.