This is a ceramic furnace as large as a washing machine, as heavy as a Honda Civic, that needs to be 6 times hotter inside than their home stove/oven/range, and the fuel (ZnO) is safe to eat in small quantities (breakfast cereal) but is toxic(?) to breathe.

I'm having trouble seeing why it would take long for a startup to start baking the clay for one of these in every home.

Zinc boils at just over 900 degrees centigrade. I guess that might make this a bit risky as a device. If, as you say, Zinc in gas form is toxic, that might be a problem.

Two by three feet and 10000 suns translates to "on a perfectly sunny day, needs a 200 by 300 feet solar collector" to produce the required heat. Actual numbers for a machine operating throughout the year will be quite a bit higher.

Speaking from experience (DIY foundry mishap involving brass), even small amounts of airborne zinc fumes are sufficient to knock a healthy adult flat on their ass for four days with superflu style symptoms.

While the goal of the project was to create a 100% renewable energy source based on sunlight, another energy source could probably be used.

This is a heat engine. Your options for renewable heat are mostly geothermal or solar.

The question is... if we connected the hydrogen to a fuel cell, is it more efficient than a steam generator?

I am skeptical. Large steam generators are very, very good.

If I was breathing 900C+ zinc gasses, I don't think its toxicity would be anywhere on my radar of concerns at the moment.

Nitpick: 3.6 times hotter than a home oven, not 6. Fahrenheit is not a linear scale, you must subtract absolute zero (about -460°F) from measurements (or convert to Kelvin) before computing ratios.

Nitpick, the modern Civics weigh about 2600-2800lbs, so it's about 70% of a Civic. Certainly the right order of magnitude though.

There's something ironic in you nit-picking about a 'modern' weight measure and then giving an answer in pounds (lbs).

2700lbs is ~1200 kg

Nitpick: kilograms aren't a measure of weight.

Lol, got me. Massive error on my part.