Not to mention that nowadays unlike the days of airbrakes the passcomm doesn't just ditch the train pipe, slamming in the brakes, but goes via the driver.
Trains still have air brakes, but on more modern stock the passcom equipment may not immediately affect them if it's overridden in time.
It's important to note, though, that this statement is more than a bit misleading for a couple of reasons. Firstly, there's plenty of stock (mainly late BR era, early privatisation, or heritage stuff used on tours and charters) where the train brakes will be applied straight away if someone pushes/pulls the alarm, and where this can't be overridden, because the passcom and braking system are still directly connected to each other. Secondly, even on stock where overrides do exist because the brakes aren't directly connected, the driver has to actively opt to override the passenger alarm (within a certain time). Although it may not apply here, occasions where the alarm would not be overridden include the train leaving a station from which it has just been dispatched, for example.
Granted, there are trains where there is a difference between "call for aid" and "passcom", but for the most part, any alarms near the doors or in the main part of the saloon will be able to cause the brakes to activate if not overridden, ie. if the driver feels this would be unsafe, or if they are incapacitated.