I was diagnosed with Asperger's at the time (in my early 30s, about 10 years ago) and this was the first time I had ever heard about this.
Apparently, there is typically a point in an NT child's development when they become aware of other people's existence when they're not there, which I believe normally kicks in at around 5 or so.
I was honestly probably in my late teens before I had that 'aha!' moment and I still don't have a great grasp of it, tbh. It can be useful, though - if somebody or something offends me, they are literally dead to me and may as well have never existed. I don't hold grudges!
I also get the idea of 'out of sight, out of mind', which is why I have a series of highly complicated systems in place to make sure I can put my hand on anything at any given time (and get upset if people - typically NTs - mess with it).
I have a theory that autistic people's lack of or struggle with the concept of OP ties in to NTs' idea that 'all autistic people lack empathy'.
I don't lack empathy per se, but I cannot conceive of feeling on behalf of another person or thing when they're not there, if that makes sense?