I think you are both correct. It is true that people who grow up poor simply have much fewer chances to learn how to grow wealth. They may be frugal and manage the little they have with care, but that kind of mindset is not enough in general. They don't get any chance to learn how to invest money properly, because they don't have any to invest. If they do put something aside, they are encouraged (by the circumstances) to keep it as safe as possible (ie cash), which is generally a bad investment decision.

You are also right in that there are plenty of people who are high earners, but never get to actually build wealth because they spend it all. This can often be the curse of living in a high cost of living area. You see all the wealthy people driving in nice cars and living in luxury homes, and this makes it difficult to consciously deny yourself these things and live (relatively) modestly but build long term wealth instead.