So this addresses a 'need' but that 'need' doesn't have a market. In fact, that shadow-market is merely a consequence of the failure other markets. The need for this service is completely dependent upon the failure of the local restaurant industry to properly manage its demand, which in turn is likely due to the failure of a real estate market to provide profitable space for restaurants.
If EITHER of these two markets are disrupted, your market evaporates. Considering how popular food blogs and food trucks are becoming, I would not bank on this.
The 'need' is that there are open seats at restaurants that don't have any open reservations, due to people showing up late or not showing up at all. This inefficiency can be looked at as unavoidable friction, or as opportunity for innovation.
Other industries in the hospitality world have their own ways around this. Most hotels have 24-hour cancellation policies and airlines have dynamic pricing. This is a new one for the (fine) dining industry, and one that will likely spawn different models for different types of restaurants.
Some higher-end restaurants do take a deposit when you make a reservation but it's not common. I find it interesting that there's so much variation among different types of services (hotels, rental cards, hotels, restaurants, theater) with respect to cancelation or no show policies. It doesn't obviously always track to the cost.
I don't think the restaurants that this is being marketed towards are competing with food trucks and blogs.
To assume that the real-estate market will suddenly become perfect is kinda ignorant considering how unscalable commercial real-estate is. And this product that they're claiming to make IS THE THING that will solve the local restaurant's demand management issue.
So what you've said basically is that IF unicorns exist OR their product doesn't work THEN they'll fail. ( A+B' = B')
Well I can remove a logic gate for you: IF their product doesn't work THEN they'll fail. (B')
If EITHER of these two markets are disrupted, your market evaporates. Considering how popular food blogs and food trucks are becoming, I would not bank on this.