"Was it really punk that showed people that they could attempt music without any musical knowledge or skill? I thought that that happened in the skiffle era."

Suppose it depends on what 'people' you are talking about? ie what generation you are and what context you are talking about! I'm in my early 50s and was 16/17 in 1976/77. I did't really know much about skiffle but punk was there as a basic antidote to the excesses of some of the progrock etc.