You are absolutely correct.
Who defines what is Rock? Lemmy, an obvious Subject Matter Expert, stated “We are Motorhead. We play Rock and Roll”. No one would disagree with him.
Now listen to another bass player- Paul McCartney. Does he sound anything like Lemmy? Absolutely not. Paul plays Pop Music.
Butthole Surfers = rock
https://www.youtube.com/watch?v=CNAkbbKycCM
Hmmm, so the term Rock and Roll was originally used in the 1950s to apply to performers like Elvis, Bill Haley, Chuck Berry, Little Richard, Buddy Holly, etc. But you would say they aren't Rock and Roll, because they don't sound like Lemmy?