Sysadmin originally meant Unix greybeards who were competent at C and could write and implement, say, Kerberos. Then in the 90s-2000s the term came to primarily refer to Windows admins clicking around Active Directory and Group Policy for large enterprises. The "sysadmins can't code" thing came grom that time and the early 2010s when all the cool startups were building on Linux and the available pool of sysadmins were largely Windows specialists. Then DevOps came along trying to get Windows sysadmins that were dabbling in Linux into modern development practices, and SRE came along trying to revitalise the old Unix greybeard style with modern software development practices.