For a man, a brothel operator, a private detective, or a drugs purveyor would make more sense for blackmail. How would a waiter in a nondescript location gain access to information that would allow him to blackmail? It's not like the hotel was placed near a military base or was frequented by diplomats or generals. Not to mention placing tapping devices in the Queen's furniture. The story, as presented, does not compute.
Karl Koch and his buddies were recruited by a KGB spy posing as a casino croupier in exchange for gambling money and cocaine.
A hotel waiter or a concierge is in the perfect position to mingle with the rich and powerful.
Upper class people use hotels for extra-marital affairs and business trips in unknown cities. You do not need a first degree connection with the military to get an in.
The bar of a famous Dutch hotel was known to be frequented by American spies and drug agents looking to ensnare peons of the local drug maffia, working their way up from there.
>>>> Not to mention placing tapping devices in the Queen's furniture.
They didn't actually placed tapping devices in the Queen's furniture. From the article:
One of his first ideas was to try planting listening devices in the Queen's furniture, he recalls, though he and his bosses realised it was technically unrealistic.
I think he knows that, but that part you quoted didn't make sense to him, since how would Van Haarlem even be in a position to access the Queen's furniture, merely working down the road from Buckingham Palace?
(At least that's what I thought when reading it anyway.)