> How does that work if a digikey order takes 1-2 days, and ordering a PCB even longer?

In this case, Hardware hackathon means wiring up the digital or analog input/output pins of an Arduino/Raspberry pi to something and writing software to control/interface with it. Perhaps with a few simple components on a prototype board

We did this once at a traditional hackathon - we took a food dehydrator and wired up a temp sensor and raspberry pi, wrote PID controls and turned it into a 3D printer filament dryer.