Here is more on the process that might make this print actually possible:

Small increments of molten metal are welded to the existing structure, creating lines of steel. By printing multiple lines, the printer will be able to create a strong, complex structure that spans the canal — printing its own supports along the way so that it can operate autonomously.

MX3D is the company responsible and here is how they describe themselves:

With our robots that can “draw” steel structures in 3D, we will print a bridge over water in the center of Amsterdam. We research and develop groundbreaking, cost-effective robotic technology with which we can 3D print beautiful, functional objects in almost any form. The ultimate test? Printing an intricate, ornate metal bridge for a special location to show what our robots and software, engineers, craftsmen and designers can do.

This really is groundbreaking and we’re looking forward to seeing more from this innovative space.

thanks to CNET for the great writeup