Brief Answer:
Usually distinguishable, but too much of a clinical diagnosis.

Detailed Answer:
I read your question and I understand your frustration. Being told to have had a wrong diagnosis after so many years must have felt terrible.

Both parkinson's and dystonia are part of a group of disorders called movement disorders. I must say that it's probably the most difficult sub-specialty in neurology because these disorders often have similar overlapping characteristics making it difficult to distinguish between each other, at times for some of them only autopsy makes a definitive diagnosis, in a considerable number of parkinsonian syndromes cases, autopsy studies have shown a different type of disorder from that previously thought.
What renders distinction more difficult compared to other disorders is that there are no imaging or lab tests to support the clinician. In recent years PET and SPECT scanning are being used in Parkinson's diagnosis, but they are expensive studies, not routine ones, not available everywhere and not present at the time of your diagnosis 19 years ago.
What I am trying to say with this opening is that in general I would be very careful to point the finger towards a clinician when it is the case of a movement disorder.

In your specific case, well typically the two disorders have different presentations and are distinguishable from each other, but also both conditions can have heterogeneous presentation, so theoretically certain distributions of dystonia could be mistaken for forms of Parkinson's where akinesis-rigidity symptoms without resting tremor prevail. They could also both respond to levodopa which doesn't help.

However Parkinson's cases at that age are very rare and the clinician must be very suspicious before making a definite Parkinson's diagnosis, taking into account the possibility of other conditions and re-evaluating the diagnosis in follow-up visits. 19 years seems a really long time, lack of typical progression in 5, maybe 10 years at most should have prompted a reconsideration of the diagnosis. So I am inclined more towards thinking an alternative diagnosis should have been thought of earlier.
As I said before though, it's too much of a clinical diagnosis. If you gave some more history details on your symptoms, the body parts involved, progression in time, received treatment and response to it, perhaps I could be more decisive. However it's the little details in the neurologic examination which can sway the opinion towards one condition or the other.

I remain at your disposal for further questions.

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Brief Answer:
Dystonia should have been considered.

Detailed Answer:
Judging from that info, at your age at the time, I must confess my first thought would most likely have been dystonia not Parkinson's disease. Even in case signs of which you are not aware of were noted on exam, diagnosis should have been reconsidered periodically.
So my initial opinion siding with the Toronto neurologists is reinforced I must say, I am really sorry.

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