I can't agree with that at all. Sterling is always involved and has run games in the past for Liverpool. He is playing a different role for City but you can see from his goals and assists record that Guardiola has improved him massively. Half the time you only know Walcott is playing when he misses a good chance.
Walcott lacks the footballing brain of Sterling, Walcott could NEVER play in a Guardiola side that focuses on possession. Walcott is not an especially good finisher - as proved by his wastefulness. The law of averages dictates that if you have as many chances as Walcott, you will score a few goals. Technically Walcott is nothing special at all. They play different positions, Sterling is not as far forward as Walcott, but has added goals to his game now.