Not sure why you are being down voted for this - we ratified GDPR under UK law, the UK Data Protection act of 2018 - until that law is repealed GPDR still applies under my understanding.
They might be simply preparing for that eventuality, and when the law changes they can quickly react. Apparently in the e-mail that the users got, they are claiming that nothing privacy wise will change, and that might indeed be true for now.
I think that also breaking the law might be easier. EU might not bother anymore penalizing them for a member that's already leaving, and UK alone will have very little force imposing penalties on an US corporation.
The UK is planning to leave the EU, so Google is planning to treat UK accounts as not part of the EU. I don't see what is difficult to understand about this.
The UK is following EU rules and regulations at least until the end of the year. After that point, it can pick and choose, based on negotiations for trade agreements or whatever other reason pleases it.