iPhone 6s button not working after screen repair

I tried to replace my iPhone 6s with a lcd replacement I bought. I turned the phone off but unfortunately did not take off the battery. I proceeded to uninstall the screen, the earpiece and camera. While trying to take off the screws for the button and the 6 screws around the phone I realized that the tools I got were cheap and the screws were in too tight and I was damaging the star of the screw. I figured that I wasnt going to be able to do this myself so i brought all the parts to a phone repair shop for them to install a new screen for me (not the one i originally had, I eventually returned the LCD screen I bought). When I got the phone back to me he let me know that it was in DFU mode and that it doesnt usually happen with phones hes fixed. The weird thing was I just turned the phone off and then on again and the apple logo popped up and my phone functioned the same again with all my things on it and all I had to do was put in my password. I noticed that my home button was not working too. (touch id/pressing the button no response to the phone). I then updated my phone thru my phone and it went on DFU mode again but then I turned it off then turned it back on again and continued to finish loading the update and then my phone turned back on and was updated. I went back the next day to get my home button fixed. The repair man told me it wouldnt work anymore because when i was taking the screen apart I was supposed to also take out the battery too and that caused my button to not function anymore. I asked him why it was only my button that didnt work and why everything else works (even 3d touch). He kept saying it was because i didnt take off the battery. Im wondering if this is the true reason as to why my button is not working anymore. Everything works but not my button just because of the battery? Is my button not able to work forever? Why was my phone going in DFU mode?