"44x17 fixed gear. So when I get off the bike and
start walking"
Walking?!?To each his own.
"I guess you can spin up the tiny gear and still maintain a reasonable (faster than walking) speed on the steepest climbs."
Not necessarily. In your case, you're turning the cranks 15 times a minute on your fixie to maintain your 5km speed. I can imagine that it would be a challenge just to stay upright at that very slow speed (about 3mph for the metric-challenged).
With a 30t triple and 28 cog at 5km, you'd still be spinning very slowly, under 40 rpms. And at such a slow speed, again the ability to balance would be an issue.
So whether or not someone can walk as fast or faster than a cyclist up a steep hill depends more than on just the cyclist's gearing. It depends on the steepness of the hill as well as the fitness of the rider.