Earlier quoted context omitted.
Please double check my wording. 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
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.