This is tricky one.
The dictionary definition of retro is as follows
Main Entry: retÂ·ro
Etymology: French rÃ©tro, short for rÃ©trospectif retrospective
: relating to, reviving, or being the styles and especially the fashions of the past : fashionably nostalgic or old-fashioned
Clearly we're using retro somewhat differently, in this case it refers to something which is actually from 'back in the day' (whatever that means!).
For me a very very general definition of retro is anything built in the mid 90s or earlier. Certainly the parts and bikes covered by scant's definition are probably retro, but as elev points out there are many bikes and parts which fall outside this definition which are still definitely retro.
Back to your original point as fine and as workmanlike as an 8 speed XT gruppo is I would not say it's retro.