In addition to your point about leverage and underlying asset volatility, certain assets are not regularly marked to market (real estate being a prime example) and so you don't experience the true volatility of the asset unless you attempt to sell it.

As a concrete example, a number of commercial real estate investors were technically insolvent in 2008-2009, with assets worth less than the balances of the loans used to buy them. They just pursued the 'hear no evil/see no evil/speak no evil' approach and marked to book (what they paid for the asset) until the market recovered. This approach is aided by the multi-year nature of commercial leases, which protects the cash flows needed for debt service (as long as your tenants stay in business).

In aggregate these factors allow professional real estate investors to consistently earn return by taking on a ton of leverage and with it huge but disguised risk. Back to the original point of the article (buying to rent), most retail investors don't necessarily have the float/access to debt to weather that volatility, and their cash flows are more sensitive which compounds that risk.