I don't believe that "makes money partially through investing" is true for a significant part of the population given that 60+% of Americans don't have $500 in savings. Though perhaps that is the point

The internet suggests that the median American household has closer to $5,000 in a savings account. Regardless, while you may see some paltry returns, savings is not really what I think most would consider any meaningful investment income. The primary purpose of a savings account is to keep a modest sum of liquid cash available for emergency purposes and not where one would want to keep most of their assets.

Wealth figures are more difficult to come by, but the internet suggests the median household has a net worth of around $200,000. However, I would venture to guess that a large portion of that is tied up in a primary household which does not normally generate any income. It is not clear how much a typical person does make in investment income.

If you are right that 60% of the population do not have investment income, that seems reasonable enough. I think it is fair to say that most people are working class. I am sure that we can agree that, under modern usage, the middle class is meant to represent something that is not easily attainable, but more attainable than upper class.

In addition to most "middle class" wealth being tied up in home equity, I would wager that the vast majority of "investments" is tied up in retirement funds. Very few middle class people have regular income from investments. Outside of Silicon Valley where RSUs and ESPPs are common, few people can rely on stocks or other equities to generate any usable income outside of retirement.