Jurisdiction is a tricky concept in cases like these where we are dealing with digital content.

AggregateIQ could have ignored the GDPR-request, ignored any rulings and keep chugging along as long as they stuck to Canada (assuming Canada was not going to side with the EU).

In short, if you're not in the EU, do not care about EU and never will, you can largely ignore the GDPR. Same as if some banana republic dictator declares you persona non grata - if you never intend to visit and otherwise have no business in that country, who cares?

The difference is that the EU is not a banana republic (opinions may vary), and so many choose to respect and accept this their judgement in cases like this to stay on good terms.