Optical devices are physically large with waveguides about 1um and and most devices are a few hundred microns in size. The Mach Zehnders that they discuss are

Optical devices are physically large with waveguides about 1um and and most devices are a few hundred microns in size. The Mach Zehnders that they discuss are 100’s of microns in physical size. So it’s typical that silicon photonics are fabricated in older nodes in part because the resolution of advanced nodes is not needed and it’s a lot cheaper to use the older nodes. Additionally the sweet spot for silicon photonics is a wavelength ~ 1.3um (1.5um) so the node resolution is sufficient for the wavelength used.