What functionality or property makes JetBrains' products an IDE while VSCode isn't? Honest question, I've never used any of their products.
Or to make things more succinct:
* VSCode is a extendable code editor (like vim, neovim, Zed and Sublime)
* Jetbrains Rider is a fully equipped Integrated Development Environment (like Microsoft Visual Studio or its direct sibling Jetbrains IntelliJ IDEA)
And while extensions are optional within a IDE (and often solely used for increased productivity), more often than not they are a necessity in a code editor to even become productive.
"Integrated" isn't meant to contrast with a plugin-based system (otherwise JetBrains wouldn't count!), it's meant to contrast with a dev environment built out of a bunch of individual tools and terminal commands run separately.