Pretty much.
They're "regular" in that their income is not high - significantly less than me, and I earn significantly less than SV folks. Nurses, journalists, people working at the bank, etc. Many don't have a degree. None come with money.[2]
My wife would bug me often to "look into real estate", and a cursory look always seemed insane - no way is it outperforming the S&P long term! Finally I decided to look at it in depth, and although I personally did not get involved in RE investing, I did see how it works.
Owning 5 extra home sounds like a wealthy person, but the reality is most of these people aim to earn about $200/mo net profit on each of these homes. The rest of the money goes into paying off the loan, repairs, saving for vacancies, property management, etc. You can make a lot if the property/rent appreciates significantly, but most of these folks do not rely on it. And then on top of that there are some tax benefits.
The trick is finding ways to finance it, and finding good deals. The typical house for sale on the MLS will not earn you money and will likely be a loss unless it happens to appreciate. So most of these folks are looking for deals: Properties that banks will not approve loans to purchase because they are in such bad condition. Say a house, if in good condition, would sell for $300K. However, it's in very poor shape and the owner needs money. His balance on the loan is $100K. You pay him $150K or $200K in cash, spend $50K to get the property up to code, and rent it out.
But who has $150K cash lying around? Not these guys. So they find rich people who'll lend it to them (including the $50K to repair) at 12% interest rates. The idea is that it will take 3-6 months to bring the property up to code, so even though the rates seem high, the absolute amount you pay on that interest is not that high. Once it's up to code, the house is worth the market rate of $300K. You refinance, pay the original lender his balance, and can either sell the house (flip), or rent it. You often get some extra cash this way to help finance the next deal.
The beauty of it all? It's low risk. These are non-recourse loans. If you miscalculated and the deal isn't going to be profitable (e.g. the cost to repair is a lot more than estimated), the lender cannot come after your home, car, bank account, etc. They only get to keep the property you bought. You get no hit to your credit.
Of course, because of this, most such lenders will want you to put in some money (e.g. $20K). And they'll want to see how you made the repair estimates, etc. They'll lend you money only if they agree with your analysis.
Real estate is really one of the few options accessible to most - as opposed to, say, a 401K that requires an employer to offer it. It's a lot safer and simpler than most other investments. If you compare it with trying to understand the stock market, you begin to wonder why people invest in the stock market at all. Within a few months of reading and studying, you'll know all you need to know to start making money. So yes, that's why so many "regular" people are in it: It's simple[1], and they don't have access to other ways of making money. Few will lend you $150K to put into the stock market. But many will lend you that amount for real estate investing.
[1] "It's simple. Buy low, sell high. It's so simple even Donald Trump can do it!" (Common refrain you'll hear).
[2] Now above I'm talking about traditional real estate/rentals. If you go into AirBnB, the profit margins can be much higher. Over $1000/mo net profit per unit. I know one guy who was probably earning less than $70K/year in California study how to do STRs, and he's doing very well now. 2 years in made over $350K net/year, and growth is rapid - he expects to hit $1M/year soon. Obviously, he's an outlier, and he did have to work hard at it (quit his real job) but everything he did is very accessible to regular folks.