This stuff happened before, or at least around the time, that .NET Core was released. So it either didn’t exist or was an extremely new option, at least.
According to the article they have been using rust in production for 1½ years, .net core 2 was released in 2017 so it definitely existed. If .net core was mature enough for their liking is however another matter.
.net core was released in August 2017. That’s basically one and a half years ago. Depending on how fuzzy the one and a half year time is, it may not have been released. Like it may not have literally been 18 months exactly. That’s all I’m saying.