You are right. Also, a desktop environment, at least from an end user’s perspective, is as part of the OS as a kernel, terminal, and its associated tools. We are just using the language that a non-techie can understand and act upon.
Either way, “operating system” is a woolly and ambiguous term that is hard to define precisely and changes meanings depending on who you ask. The common denominator in common non technical English seems to be “software that allows you to manage you hardware and applications”. If that is so, yep, Plasma fits the bill.