Not true. It’s not uncommon for a toddler to find a private place to defecate without being told. There is an innate sense of a need for privacy for some things.
I will now point you towards mainland China, where kids as old as 90 will shit in the streets. I’m not kidding either (although this is changing in the larger cities.... slowly)
A private place is often a good idea, so predators wont eat you when you are so vulnerable. There are many places where nakedness is the norm and the naked people will probably still search for private places when defecating.
Are you sure that's not learned behaviour? I've only ever seen it in older kids. I'd hypothesise it follows "expert" advise that suggests what I consider very late potty training.
I've spent time around more babies & toddlers than most people see in a lifetime (sometimes >50 per day; probably no more than 2000 per year though; for >decade; [working at baby & toddler groups]).
Could you say if this was a fixed behaviour (ie they always sought a private place to poop), had a fixed location (eg always behind the couch), if you used a potty at the time, and exactly which month it appeared at so I can note it with my own observations. Cheers.