That's toilet training - learning to unconconsciously 'hold it in' until you're in a suitable place to let it go.
Actually, I've been doing a bit of reading into this, too, and EC or Natural Infant Hygiene does NOT start out with the idea of teaching the baby to 'hold it in' and there is a definite difference between learning to 'let it go' and 'holding it in'. Initially, you're getting to know when they want to 'let it go' (which is from birth, they know when they want to let it go, Western mothers just don't know how to respond to it). It's later on that they learn to hold it in - it's the 'holding in' that they think they need to do when we TT them at a later age that contributes to kids getting upset and having pressure on them to let it out when they've lost touch with how to do it.
What we're actually doing when we TT toddlers is getting them to RE-learn toileting, because they have become used to going to the toilet in their pants and ignoring their own bodies' signals.
What happens is that babies usually first become night-continent, because a hormone is realeased during sleep (and also in other conditions like dehydration) that inhibits urine production - babies feel the need to wee when they are in lighter sleep and will usually stir or make a noise at this point. Then, within a short time they start to do a poo in the morning and it progresses from there, till the average age of about 1 year when they are continent enough to 'let it go' AND 'hold it in' for a short period.
I haven't done it yet, but I will be giving it a go for nights with DS (he's continent during the day), as we co-sleep and it would be nice to have him out of his PeaPods entirely
I've been reading 'Diaper-Free Babies' from my local library and it's really good in explaining the physiology of elimination.
I'm reading up on it now so that I'm prepared from day 1 with my next child, whenever that one comes along!
I'm not scared of it, especially now that I have a far better understanding of the mechanics and techniques that people around the world use. It just makes so much sense, and I intend to be present for the next child enough to merit giving it a go. Another compelling reason in my family to have larger age gaps
It would be nice if a current affairs show did a story on this type of toileting, but it wouldn't happen cos they'd think they'd have to boost ratings by sensationalising it and rendering us free-thinkers as 'weirdo hippies' - they wouldn't even say the bit about millions of people in the world (actually, try billions!) learned to toilet in this way.
So, WRT the OP, yes it IS true...and has been true for millenia