Checking if Type or instance implements IEnumerable regardless of Type T

In addition to Type.IsAssignableFrom(Type), you can also use Type.GetInterfaces():

public static bool ImplementsInterface(this Type type, Type interfaceType)
{
    // Deal with the edge case
    if ( type == interfaceType)
        return true;

    bool implemented = type.GetInterfaces().Contains(interfaceType);
    return implemented;
}

That way, if you wanted to check multiple interfaces you could easily modify ImplementsInterface to take multiple interfaces.

To check if some type implements IEnumerable regardless of T one needs to check the GenericTypeDefinition.

public static bool IsIEnumerableOfT(this Type type)
{
    return type.GetInterfaces().Any(x => x.IsGenericType
           && x.GetGenericTypeDefinition() == typeof(IEnumerable<>));
}

The following line

return (type is IEnumerable);

is asking "if an instance of Type, type is IEnumerable", which clearly it is not.

You want to do is:

return typeof(IEnumerable).IsAssignableFrom(type);