Why is Dart not multithreaded?

By design, Dart is a single-threaded programming language . That’s mean we have asynchronous code across application. When a program starts, it creates something that is called Isolate. When isolated created, the microtask manager executes all events asynchronously.8 Eki 2019

In Documentation, Dart is Single Threaded but to perform two operations at a time we use future objects which work same as thread. Use Future objects (futures) to perform asynchronous operations. If Dart is single threaded then why it allows to perform asynchronous operations.

Each Dart isolate has a single thread of execution and shares no mutable objects with other isolates. To communicate with each other, isolates use message passing.