With apologies for my earlier response - it's true, but not entirely helpful.
Most of us have a 'lowest comfortable cadence'TM - if your gearing and power to weigh ratio allow you to climb at that or above you'll be fine. If not then lower gears or an improved poewr to weight are called for.
There's a huge psychological component though. As you say, lower gears just mean you can get by if you need to go slower, but it soon gets to the point that walking is faster.
I've done the same big long ride with big long hills (The Elenith 300km Audax) on both gears and fixed. I had to walk pretty much exactly the same really steep sections of the ride (Devil's Staircase, road out of Pont-Rhyd-y-Groes, some other bit that I can't remember) whether I was on fixed (67") or gears (despite having a lowest gear of 30x29).
The human body is still efficient enough at 25rpm, so the choice of gears isn't that important; if you can push out
x00W of power then, at that slow speed, it doesn't really matter what cadence you're doing. If you ride fixed regularly then you get used to putting out near that power at very low cadence as the alternative is a (marginally) slower walk.
Psychologically you may think you need lower gears, but when you have them you only want even lower gears. When you don't have them you just get on with it.