This is all your opinion. As someone that literally lives in Zone 2, I rely on Citymapper literally every day and in my actual experience as a Londoner, Citymapper beats Google every day. If Google were better, I'd use Google, but it's not.
Also, another major benefit of Citymapper over Google. Citymapper does a much better job of orienting you when walking. Google Maps seems to be slow, or maybe have UI issues when I'm trying to work out where I am and which direction I should be walking in. With Citymapper I never have an issue, it seems to use the IPhone compass functionality better than the Google Maps app.
The other major, major issue with Google, is the Directions UI on phone just seems half baked, like it was an after-thought. It has never felt nice to use and I'm often struggling trying to work out how to make it do what I want. I struggled when I first moved to London getting around until a co-worker advised I use CityMapper and then I never went back, it actually made me travel around London more because I felt so much more comfortable with how to get around.
E.g. On my phone I just tried to make Google show me a route from my home to a pharmacy in central London. The result: "Can't find a way there."
Google Maps can't, at least on my phone (11 pro) with the latest version, show me routes involving buses AND trains. It only shows me trains and not buses. If there is a way to make it show both it doesn't make it obvious.
In Google Maps I can only click on one option, walking, trains, car, etc. There is no option to just show me all methods and then compare them. This is one of the things CityMapper does incredibly well, it shows you every option possible and does so in a very nice presentation. Google Maps assumed you already know which SINGLE method of transportation you want to use and you have to choose it to begin with, which in my opinion is a poor way of handling this situation.