They cannot aggregate or anonymise the data locally on your device, because that requires combining it with data from other devices. They can only water it down a little, just like google does with Floc or whatever they call it now.
The difference between this and the minor act of selling out that is their main source of income is enormous and seems too obvious to need commenting on. The difference between this and something more comparable such as sponsored links in Pocket is indeed that they are starting to collect audience data to enable it. It's a small start, but it's a major departure from the already somewhat distasteful situation people were accustomed to.