Calling time-sharing “PaaS” makes essentially the same mistake as calling pre-IaaS systems “cloud”.

First, that’s not what they were called at the time.

Second, those older systems aren’t what either term actually refers to.

(Selling time on a mainframe would be closer to IaaS than PaaS anyway - in fact IBM’s VM/CMS even used the VM abstraction, which compute in most IaaS environments depend on.)

Your position only works if you ignore the actual extant definitions of what “cloud” or “PaaS” actually refer to.