However, they absolutely did business servicing the EU before and gathered an enormous amount of personal data of EU citizens by doing that.

As such I'd be surprised if they simply declare their service closed at one time and instantly lose all obligations for the data they collected in the past.

As an analogy, if you paid for some service in advance, the service took your money and then, instead of providing the service, spontaneously declared to end their relationship with you, you'd probably have good chances if you were to sue them.