That seems a rather optimistic assumption, given the historical way data brokers and those who use them have operated. Plenty of businesses, including some household names, have been caught with their hands in the cookie jar on this one before. No doubt plenty are still doing it and hoping not to get caught or that any penalties will be small enough to be worth it.
As I wrote earlier, the issue here is that because they have no direct relationship with people in their data lake, there's no way for them to know with certainty that the email address associated with a person belongs to that person without some form of additional validation.
There are few ways to know anything with true certainty unless someone in your organisation personally knows someone you're dealing with. It is more about being reasonable.
If an organisation maintaining large amounts of personal data about people without their consent can't find a reasonable way to verify identity and allow the data subjects to exercise their rights, the GDPR-esque solution to the problem is to shut that processing down entirely until the organisation can get its house in order, or permanently if it can't find a way to do that. If that kills the data broker's business model, maybe they shouldn't have been using that business model in the first place, or should have discontinued it when the GDPR came into effect.
Allowing the organisation to deny data subjects their legal rights by hiding behind the verification obligation is at best against the spirit of the law but probably against its letter as well, and certainly justifies a regulatory investigation if it's being done systematically by a big organisation that should know better.
