The engineer vs. an engineer?

[Torsten]

Hi, can you please tell me why in the following sentence the definite article has been used instead of 'a'?

PC Repairs London is run by Mark McGlashan the former IBM Engineer.

I mean, why not say '.... run by Mark McGlashan a former IBM engineer?

Many thanks,
Torsten

TOEIC short conversations: Talking about expense reports

[Foah]

Hi Torsten,

as your fellow countryman I´d say that if you would refer to Mark McGlashan as to the former IBM Engineer you´d rather make sure that If there were another Mark McGlashan (probably a dish washer or teacher or salesman, whatever) you mean exactly the Engineer. But that is what my mother tongue would suggest, only.
_________________
"Ho ho!" said the clown

[Abeer]

I am waiting.