When .NET Standard 2.0 was introduced, .NET Framework 4.6.1 and 4.6.2 were already out. Because the .NET 4.6.X Sku was the most stable release back then, the .NET Foundation decided it was worth breaking the theme of the .NET Standard a little and declare .NET 4.6.1 and 4.6.2 as .NET Standard 2.0 compatible even though there were some exotic APIs missing.
This was done under the assumption that these APIs are not that commonly used, such that incompatibility would not make much problems. While the underlying assumption holds true to this today, Fido2 uses several of these APIs.
See the official documentation regarding this issue for more information: https://github.com/dotnet/standard/tree/master/docs/planning/netstandard-2.0#net-framework-461-supporting-net-standard-20
For us this means that when consuming this package in a .NET 4.6.1 or 4.6.2 targeting project, at runtime you will see TypeLoadExceptions for types like "ECPoint" - essentially breaking the functionality of this library.
Because NuGet doesn't give us the ability to reduce these frameworks from the .NET Standard restore graphs, we have to fall back to MSBuild errors preventing you to even build a project with the offending configurations.