Lol? Everything has improved from coaching to individual performance to more advanced tactics and deep playbooks. Meta has advanced new smokes have been created and learned same with pop flashes and nade stacks. Things that used to work in 2015 would never work now vs top teams. Teams have learned better teamplay and better positioning as well and also practice more on average than they used to.
Teams have better staffing around them and are more professional, theres more organizations, money, sponsors, and organizers in the scene.
What makes you think that statement wouldnt be true? I guarantee you if you asked any pro that is still in the top 20 from 2015 they will 100% say they improved individually in their role and that the overall level of competition in CS has greatly increased.