Why does expander have a constant for a template part name, but no attribute?

[adegeo]

I discovered this constant: ExpanderToggleButtonTemplateName="HeaderSite"

https://github.com/dotnet/wpf/blob/main/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/Expander.cs#L349

Which is used in the template:

https://github.com/dotnet/wpf/blob/main/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/Expander.cs#L306-L310

But it's not being declared as a template part. Is this a bug?