“Unmetered” means “You will not be charged under normal circumstances based on the measurement of the data you use.” It does not mean that your traffic is literally not measured.
They don’t put a specific hard limit because doing so both limits their own flexibility as a service provider and creates a target for abuse by users.
For some definition of "normal circumstances". Being a bigger user should fall within it or that's not accurate advertising.
Some places will offer a choice between faster metered and slower unmetered. That seems like a good compromise to me. A nice big link should cost the host a single digit number of dollars per 100Mbps, so it's not hard to find an option where everyone is happy with the speed and pricing.
If you want a contract that has every term and circumstance negotiated up front, you’re going to need to speak with Hetzner’s business development team. You’ll also need to be a bigger fish than a single hobby developer.
They don’t put a specific hard limit because doing so both limits their own flexibility as a service provider and creates a target for abuse by users.