I think the headline here is a bit clickbaity. The post-EU UK has adopted the EU's GDPR legislation wholesale, and Facebook explicitly says that data will not be treated differently than that of EU citizens.
So while this might "avoid" EU privacy rules in some stretched interpretation of "avoid", in fact it will be avoiding them by applying an identical set of rules. That is, unless the UK government changes its data protection legislation at some point, which is another kettle of fish.
I don't think it's another kettle of fish, I think it's exactly what this article is about — the only reason for them to make this change is as a pre-cursor to a divergence in the UK/EU privacy laws (EU further strengthening their laws after Brexit, or UK weakening theirs).
They've given a vague statement to defend this being necessary — "Like other companies, Facebook has had to make changes to respond to Brexit" — even though Reuters point out Twitter as an example of a company _not_ making a change like this.
Also, Facebook haven't explicitly said that they won't treat data differently to EU citizens, but that "There will be no change to the privacy controls or the services Facebook offers to people in the UK", which is subtly different.
I agree that preparation for potential divergence of the law is a major reason for this change, but even if that doesn't happen it still makes sense from a jurisdictional point of view. Having interactions between a US company and UK citizen be beholden to EU law enforcement, when neither of those countries are part of the EU just complicates things for no good reason. There are likely other complications regarding movement of data between UK, EU and US that can be simplified by applying those equivalent UK-GDRP laws on movement of data, but now just between UK and US.
The UK GDPR is not exactly the same as the EU GDPR.
To give one example, the European Data Protection Board (EDPB) has no authority to bind the ICO.
The recent fin of Twitter by the Irish DPO was subject to that mechanism (Ireland tried to fine Twitter, the EDPB said the fine was too low and it was increased).
So while this might "avoid" EU privacy rules in some stretched interpretation of "avoid", in fact it will be avoiding them by applying an identical set of rules. That is, unless the UK government changes its data protection legislation at some point, which is another kettle of fish.