So I had to look this up, because yep, that is a great nest in pigeon terms and I believe one of two reasons this is such a solidly built one.

1. Difference in species This is a wood-pigeon. You can tell by the white mark on their neck. As the name suggests they are typically found in woods, parks, wherever are trees. As such they actually build decent nests, compared to the rock pigeon with the grey stripes across their wings. Rock pigeons used to breed on ledges along cliffs, making nestbuilding superfluous.

2. They stole that nest from a different bird. Apperently this is something pigeons do. In this instamce however I think le pigeon built it themselves, as the nest matches the description of a simple nest compsed of twigs laid out in a circle.

I hope all went well for the pigeons. This still looks like a hazardous spot for a baby chick to wander off.

I was noticing that too, but my thought was, maybe it's reusing a nest built by a more competent bird. But I don't know if that's a normal pigeon behavior.