You're missing the conflict at the core of this business model. It motivates the provider to take as long as possible without souring the business relationship, because the faster they work, the less they're getting paid. So the unlimited part is basically marketing BS.

Did you look at the site at all? He clearly advertises a 48 hour average turnaround time.

Prolly a limit on how quick it is done therefore a real limit on designs per month. A bit like “unlimited” shared hosting plans.

Makes sense. Seems a bit like false advertising. Unlimited requests in this context doesn't really tell you how many design hours you are buying.

Unlimited isn’t possible because of how time and limits works, right? Is there ever an opportunity to market as unlimited without it being false?

Unlimited can be used honestly when it's practically impossible to reach the limit. For example if you are offering unlimited drink refills at a restaurant that is fair advertising because it's really unlikely your whole supply of soda syrup will be drained.

In this case it's clearly not viable because someone could easily hit a limit for design requests where it's just not possible to fulfil them on the budget. ~$500 is nothing like a months worth of unlimited design requests even if you only make a few requests per day.

It's more like subscribing to a $467/month plan were you can request several designs per month. It's not unlimited, it's limited by time, the longer a request takes, the less requests you can make per month.