Binding or x:Bind in Style Setters in WinUI 3 v1.2

[aturnbul]

Binding in Style Setters has been a wished-for feature in WinUI 3, at least for me. By chance (err, mistake) I noticed recently that x:Bind appears to work just fine in something like:
<Setter Property="FontSize" Value="{x:Bind app:App.ShellPage.RootShellFontSize, Mode=OneWay}" />.

The equivalent Binding expression <Setter Property="FontSize" Value="{Binding RootShellFontSize, ElementName=shellPage}" />
does not., as Binding is not supported in Setters according to the documentation. Is x:Bind supported in this case or have I made a mistake?