It is. Custom roms modify very little of the code and they are all based on aosp(it is open source but google controlls the changes). The whole point of aosp is to create the illusion of choice but if you really want to avoid using google spyware you have to give up on most apps or go to extreme lenghts to use an alternative. The grapheneos project is really cool and usefull but it only patches the inherent (intended)problems of android and doesnt provide a real solution.