Now I get it... it would use ntdetect.com from the boot drive normally just like it would ntldr. I guess thats so because XP's ntdetect.com can detect previous versions of Windows like its ntldr can. For example, 2000's version of ntldr can't load XP, etc.
If only NTDETECT.COM was embedded to NTLDR as well... one simple file to troubleshoot boot problems