My friend used to live in a building like this. It is assumed that everyone else in the building is equally rich and also have their own floor, so they would be able to ride up with you as you would with them. The problem now with air BNB is that you don’t really know who’s supposed to be in the building or not as many of the residents list them when they are away, which is often(either visiting their other properties or travelling recreationally/for work).

These buildings also change hands, whether between family members, or just outright sales, more so if a unit in the building is currently for sale, someone could access the building by showing interest, not even with the real estate agent but just telling the doorman you have a viewing. You wouldn’t have to hack the elevator at all and I doubt this perpetrator did. It would take a pretty vigilant doorman to stop this/determine the identity and purpose of all visitors.

Most of the doormen are there to provide additional services to the residents, like having a cab waiting when they are on their way out and other small conveniences, not so much as gate keepers especially since 99.99% of the people entering are no cause for concern.

You're less likely to get caught using a key than breaking a window.