Agreed that enlisting outside help is rare. But maybe the reporting is just a bit garbled and the person in custody is also the person in the cctv. So we are not talking about 2 potential suspects but one, who is in custody but was caught on that cctv. But the police won’t confirm that until they have a bit more concrete evidence and don’t want to “lead the public” as it were by giving away too much info about him at this stage (especially if they have not charged him as that gets into the realm of total by public and potentially compromises any future trial if they, for instance, released the cctv of him actually attacking her. Which they will save for any trial).
It might be the only image the police have of him at this stage and they have released it to try to get more info from the public regarding his movements that night to build their case.