I think it's poor wording of the title. But then I'm not native speaker, so forgive me. What you're saying is that your playing has improved not because you left a band, but because you started doing something new. So it should say "Did your playing improve once you started doing something new?" and then I can guess the answer would be "sure, yes" for most guys. If the band naturally evolves, then you should have opportunity to try something new within existing band, so leaving band is not necessary to improve.
When my band was dissolved, I had period when I didn't feel like playing, so obviously my playing did not improve. But then I joined new band and I was motivated, invigorated and I have indeed improved my playing.