Not sure why that is amazing. I met lots of folks who were on long tours that they started as non cyclists especially on the Trans America. My two companions on the TA had almost no miles under their belt at the start and one was never a cyclist previous to the TA. They both did great. Being young and in generally good shape helps but even being older of somewhat sedentary doesn't mean someone can't start a coast to coast trip if they either train a bit of take it easy for the first 10 days to 2 weeks.Read my second posting. As to my first one, yes, what you describe can be done and has been done but that still doesn't make it a good idea.
One difference is in your example, your non-cycling companions had you as a guide to both bike choice and riding. Based strictly on the OP, these guys have absolutely no knowledge of bikes and anything related. I'm sure they can and will learn but, at first blush, it really did sound like a poorly thought out idea.
I'll agree with DaveB. This action of waking up one morning and deciding to ride across the US has been done many times and will be done many more times. Probably most do OK. But I still don't think its a good idea. I don't climb mountains. If I decided to climb Everest tomorrow I would need to be put away in a locked cell. Maybe not quite the same, but its better to have experience at something before jumping into the deep end. I'm pretty sure the US will be here for a few more years. You can ride across the US 5-10-15 years from now. What is wrong with riding your bike for five years and learning about bicycling before riding across the US? Get some experience. Whether you ride next summer or ten summers from now, it will still be memorable.