It's so ridiculous because they're clearly not terrorists. The fact the police are using the law to arrest people for supporting Palestine generally, or for slogans like "Plasticine Action" just shows they're using it as a nice excuse to suppress dissent because their position on Gaza is indefensible.
I do always get a laugh when these articles talk about Sally Rooney being at risk of being arrested though, as she is an Irish woman living in Ireland who cannot be arrested under UK law unless the British decide to invade County Mayo. Her article in the Irish Times is great at pointing out how selectively the British government is enforcing the terrorism act:
The arrest of a protester in Belfast surely represents a particularly egregious example of political policing. When a storm damaged an infamous loyalist mural in north Belfast last year, rebuilding commenced immediately, and the wall is now once again emblazoned with the iconography of the Ulster Volunteer Force. No arrests were made on that basis, nor has the mural been taken down, though the UVF is a proscribed terrorist organisation responsible for the murders of hundreds of civilians. Palestine Action, proscribed under the same law, is responsible for zero deaths and has never advocated the use of violence against any human being. Why then are its supporters arrested for wearing T-shirts, while murals celebrating loyalist death squads are left untouched? Can the PSNI explain this demonstrably selective enforcement of anti-terror law?