Can you still be Middle Class if you are in the Investment Class? Yes, I think so. Being in the Investment Class means you can live off your investment earnings, like dividends and capital gains from stock holdings or rent from property holdings. If you have enough in investments to generate the income to support yourself you may not be "rich" but you can be financially independent.
The key to effective retirement planning is to outline how much income you need to support yourself and use that amount to target how much in investment assets you need to build. If your needs are modest and you can live on a modest income you won't be rich but could say "I AM in the Investment Class!"