Yes, the fact that contrary to what the title claims, at this point there is no transistor working at petaherz frequency at all. All there is, is a promising new technology.

Maybe they should have mentioned that interconnect on a chip cannot handle these speeds.

This is a laser controlled device. Even the terminology of "interconnect" is not really applicable. Your best hope is an optical waveguide coupled to the device, definitely not a metal line. It's not even a transistor in the traditional sense really.

This has limited applications. It doesn't have a viable path to being used in a CPU or GPU. So we're not going to see a zillion-fold increase in compute speeds from this. Maybe some physicists find it useful for an experiment, but the average joe won't notice anything different about the world.

I'm curious if this move by TSMC can use this research?: https://spectrum.ieee.org/microled-optical-chiplet

They seem like related attempts at creating optical processors.

OP's link is about a photonic transistor using graphene. Your link is about making interconnects using individual LEDs and fibers in parallel instead of putting multiple wavelengths on one fiber. They are only superficially related.

I thought that the speed of light limits the max possible frequency to the sub THz range, at current chip sizes.