Being a journalist doesn't make you immune from arrest. It just means that you have a good reason to be in a place to make a public record of what's happening, so you can't be arrested for petty reasons. The idea that the HK police get "confused" by student reporters or uncredited press in a way that this new system will fix is a joke. Anyone can wear a press vest. It's only when you get close enough to check a press identity card can you see whether they're official or not. That won't change with the new system. A police officer won't know by looking at someone from a distance whether they're accredited press or not.We all remember that amid the social unrest in 2019, some Hong Kong reporters, "fake journalists" and foreign reporters became accomplices as they hindered police operations in the process of law enforcement, verbally attacked police and government officers, and sent police-related information to rioters.
There's also no need to take accreditation out of the hands of the HK journalist groups because they don't accredit just anyone who asks for it.
When the HK police talk about "hindering police operations", what they mean is being closely watched so they're worried they may have to pay a price for heavy-handed tactics. They don't want that, they'd prefer the situation in mainland China where the police can do whatever they want because there's no independent reporting.
The reason behind this move is to give the police an excuse to arrest journalists present who can report on protests and/or police brutality as being taking part in an "unlawful assembly". It won't matter if they're just standing by watching, the fact that they're there at all will allow the police to intimidate and arrest them.
The CCP has already made it harder for foreign journalists to get into or stay in Hong Kong. The next step will probably be to refuse to recognise independent media organisations and only give accreditation to establishment newspapers and TV stations who can be trusted to not closely film the HK police when they're abusing their authority.