I have a windows 10x64 1909, I let the default settings all freshly download. But it's the same.
"CheckSysFiles" is automatically run when Create ISO is selected...So is "File Cleanup" to remove unwanted System Files...If you do not want the creation of the actual ISO - then right below "Rufus Downloads" there is a small box the expand "Hidden Options"Either way the Final Build should include "Create ISO"
But you just pulled the correct variable with cmd test and you also just confirmed the results in script.project....With the default value of x86 - the checksystemfiles.exe that would have been extracted would of been x86 version and would of worked just fine on x64 Host OS...CheckSysFiles - checks for & adds Missing MUI & MUN as required for System Files...