Exactly - if you take too broad a view, then the protesters having arms and fingers could be considered devices for 'locking on' to something.
Personally I think this has generated renewed interest and publicity for the republican movement, which means in a way, the protest has been a success due to the police action.
The law says there's an offence "attach themselves to another person, to an object or to land" — which could include holding hands — but for "being equipped" it needs to be an object: "A person commits an offence if they have an object with them in a place other than a dwelling with the intention that it may be used in the course of or in connection with the commission by any person of an offence under section 1(1) (offence of locking on)."
I'll not be at all surprised if later this year the Metropolitan Police arrest protesters from linking arms while surrounding a building or blocking a road.
Personally I think this has generated renewed interest and publicity for the republican movement, which means in a way, the protest has been a success due to the police action.