If you run the demo project provided with the library : 20.CustomPreview ( CustomPreview.dpr ) and open the file FlexCelVCLNT\Demo\Delphi\Modules\20.Reports\10.Getting Started Reports\Getting Started Reports.template.xlsx, you can see that the custom preview does not bold the header text : "This sheet was printed by <#RealName>", however, Microsoft Excel does it.
I am using TMS FlexCel for VCL, FMX, version 7.14.0.0 ( I think that is the lastest ) and Delphi 10.4.2.