"In 2022, 85.4 million motor vehicles were produced around the world"
Source: https://www.acea.auto/figure/world-moto ... roduction/
If you conservatively estimate that two humanoid robots can be manufactured for the same cost and difficulty as one car, then we have the theoretical ability to make 170 million of those robots yearly.
Over 1 billion humanoid robots would exist after six years of sustained production. So the prediction could come true given some reasonable assumptions about the development timetable for humanoid robots, their popularity, and how many industrial resources we devote to them.
However, I think there are many other reasons why the 1 billion mark will be missed, though I don't have time to go into it now.