On the API page we mention:

AggregationState. Contains information about the current request and rendering process. It is mostly initialized by the Aggregator filter.

The Aggregator filter does not initialize this object. It uses this object to collect content for the request. The object is initialized by the ContentTypeFilter.

On the Filters page we mention:

Additionally the AggregationState is initialized and populated with...

But it's under the wrong filter. This is the job of the ContentTypeFilter not the Context filter.

This should be updated and back-ported to all previous documentation.