Is this a reference to something? Humans cannot eat straw, straw is made of lots of plant sugars like Cellulose (probably mostly cellulose). Cellulose is a 12 carbon sugar like Sucrose but your body can't unfold it (mutorotate) due to an extra Hydroxide group sticking up that keeps your sugar-digesting enzyme from fitting around it.

Cellulose stays intact in your gut and in long strands that physically rake the interior of your gut and bind up your digestive contents. Yes, its 'fiber'.

All vertebrates lack the ability to unfold cellulose. Some animals have retained special bacteria in their gut that they house in a special organ called the cecum. This bacteria requires lots of time to break down cellulose. Animals with a cecum that can break down cellulose have a VERY slow digestive system. Some have multiple stomachs, like a cow. Others practice coprophagia, the practice of eating feces. Rabbits and deer eat their first couple bowel movements of the morning in order to give bacteria time to break down cellulose. They also need to do this to help retain that bacteria.

Our guts move food through relatively quick. Much too fast for cellulose to break things down. This is in part because we stand upright and are subsistance runners but also because we are omnivores. Mammals that eat meat tend to have extremely fast digestive systems to prevent necrotizing (flesh eating) bacterias in our food from infecting us.

TLDR: It is highly unlikely the man lived off of eating straw.