Then they'd be wrong. .net was released in 2000; Mono was 1.0 by 2004, and IIRC was usable as early as late 2002. Put another way, for almost the entire lifetime of .net (19 out of 23 years), C# has been usable on linux.
Yet the open-source community didn't build anything useful at scale on mono/C#. The C# community has still little to compete with Java when building large scale backend applications. I will love to have alternates to Hadoop, Spark, Kafka and other top Apache projects.
C# could have been much more if Microsoft collaborated with other open-source projects instead of actively fighting and killing them by offering the same functionality natively. All this for having a much tighter control over the ecosystem.
For most of its history, Mono was buggy, lagging behind, and slow. It was only ever a solution of last resort, if you absolutely had to migrate an existing Windows app to another platform.
Microsoft tried to push it a bit once they bought it for cross-platform mobile dev (back when Windows Phone was still a thing), but that never took off to any extent either.
