I think the point is less about how to specifically make 700 glasses of orange juice in the 2 hours between 5 and 7am and more about forcing a supplier to come

I think the point is less about how to specifically make 700 glasses of orange juice in the 2 hours between 5 and 7am and more about forcing a supplier to come up with a meaningful quote at non-zero cost, presumably to say "actually never mind, it was a test" or just ignore the effort. Unless this is service they already provide: even if they only have to ring round juicer machine hire in the area and figure out the logistics of getting it to the site on time it's not free to the venue. It's a way to offload the cost of your internal requirements onto each supplier, and if fact is more punitive on better suppliers who make the effort to allocate resources into coming up with a quote for a service you don't even want.