IC
Это ж ни о чём не говорит. Могли и вчера добавить.
Note that this process can lead to the creation of a delegate to an extension method, if the algorithm of Method invocations fails to find an instance method but succeeds in processing the invocation of E(A) as an extension method invocation (Extension method invocations). A delegate thus created captures the extension method as well as its first argument.
https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/conversions#method-group-conversions