St Patrick’s in NYC does not have metal detectors, but it has a lot of ushers and I suspect plainclothes security to deal with the occasional protester, mentally ill person etc. I feel confident that such a person would not make it into the sanctuary. Most urban cathedrals I have been in also have a uniformed guard right in the church worship space, granted he may be closer to the front door than the sanctuary, but I have seen them move closer to the sanctuary when Mass is in progress.
I suspect this was just not expected in Canada and took people by surprise.