Yes, you can write a non-trivial .NET application on Linux. But if you take a non-trivial .NET application that runs on Windows, the odds are low that it can easily be ported to Linux. And there are almost no non-trivial .NET applications that weren't originally written for Windows.
The result is that if you work with .NET, you're going to be pushed towards Windows.
It the wording of the announcement (taking them in good faith) that applies to applications using .NET Framework. .NET Core should be 100% portable to Linux/Mac/wasm.
.NET 5 should supersede both Core and Framework IIRC
I have been hearing announcements about how Microsoft was working to make .NET code portable ever since Mono was first started in 2001.
In the years since, I've encountered many stories that attempted to make use of that portability. All failed.
I've seen the promise of portability with other software stacks, and know how hard it is. I also know that taking software that was not written to be portable, and then making it portable, is massively harder than writing it to be portable from scratch.
So, based on both the history of .NET and general knowledge, I won't believe that .NET will actually wind up being portable until I hear stories in the wild of people porting non-trivial projects to Linux with success. And I will discount all announcements that suggest the contrary.
When you're trying to write new code, you run into a problem and look for a library that solves it. But all of the libraries that you find are Windows First, and it is not until after you're committed to them that you sometimes discover how they are Windows Only.
So yes, even in a new project, there will be a pull back to Windows. Because virtually nothing is truly written from scratch.
It’s no more of a problem with .Net Core than it is with Python modules that have native dependencies, or Node modules with native dependencies.
You’re not going to mistakenly add a non .Net Core Nuget package to a .Net Core project. It won’t even compile.
Of course you can find Windows only nuget packages for Windows only functionality like TopShelf - a package to make creating a Windows Service easy. But even then, I’ve taken the same solution and deployed it to both a Windows server and an AWS Linux based lambda just by configuring the lambda to use a different entry point (lambda handler) than the Windows server.
You can even cross compile a standalone Windows application on Linux and vice versa.
I use a Linux Docker container to build both packages via AWS CodeBuild.
Would you also criticize Python for not being cross platform because there are some Windows only modules?
Yes, you can write a non-trivial .NET application on Linux. But if you take a non-trivial .NET application that runs on Windows, the odds are low that it can easily be ported to Linux. And there are almost no non-trivial .NET applications that weren't originally written for Windows.
The result is that if you work with .NET, you're going to be pushed towards Windows.