From their FAQ:
Will Fleet replace any existing JetBrains IDEs?
No, it will not. We are fully committed to continuing active development of our IntelliJ-based IDEs. With Fleet, we’re aiming to offer an alternative view on how an IDE can be organized, and this is impossible to implement within our current product line without going against the expectations of developers currently using those other IDEs. This means Fleet will co-exist with our established products.
Additionally, Fleet heavily relies on the IntelliJ code-processing engine for its smart code-editing features, such as project-aware and context-aware code completion, navigation to definitions and usages, refactorings, on-the-fly code quality checks, and quick-fixes.