My take is that when MTH issued their UP heritage series of locomotives...someone deemed that...the plain old UP armour yellow and harbor mist gray paint livery was, indeed, part of UP's heritage. After all, it is probably the most iconic paint scheme still in existence today. So...like the other heritage locomotives...they produced a "heritage set" cataloged as 20-2774-1. Why they chose UP 8321 as the engine number...I do not know...but, they did.
I don't think you could get a separate sale UP 8321 prior to issuance of the set.