Thread safety of Datagram send and receive

The package documentation claims that `send` and `receive` operations on the same socket are not thread safe:

> The proper programming model is that one Socket is handled by a single thread. If multiple threads use one Socket concurrently, unexpected things would happen. There is one exception for multiple threads vs a single Socket: one thread reads data from a Socket only and the other thread writes data to the Socket only.

In a [discussion on StackOverflow](https://stackoverflow.com/a/61049133/9198613) though the opinion was brought up that in POSIX, `send`, `sendTo`, `receive` and `receiveFrom` are defined as atomic operations, making them thread safe at least for Datagram sockets.

With this library mostly being a thin wrapper around the Berkeley sockets API: Are operations on the same Datagram socket atomic? If not, why?