I think the point being made is that traditional signalling is designed in such a way that most failures will be right side, or protected wrong side, unless impracticable to design it that way.
For example, a broken track lead, breakdown of an IRJ, or power supply failure will cause a track circuit to show occupied. An axle counter smashed off a rail, or a cable cut, will cause it to show occupied. A black signal will put others back. A failed electromagnet will result in a driver receiving a horn regardless of signal aspect. A failed crossing barrier should end up down, not up. Go through any diagram and imagine a contact failing, you'll probably find some sort of contingency made for that.
I have a huge amount of respect for the designers of this stuff, especially interlockings. They're almost beautiful in their intricacy and completeness.
I'm not saying signalling is infallible, it's not. But it generally does a very good job of making sure things don't go catastrophically wrong if it can be helped, or two+ faults need to happen in order for a wrong side failure.
I'm sure this is something that's been thought of, and I just don't know enough about it to see the failsafes, but it does seem less reassuring.