Possibly they were referring to the fact that shares held in a margin account can be loaned for shorting, and you may be essentially holding IOUs. Margin account owners can request their shares not be shorted, but there's no guarantee this request will be honored. Shares held in cash accounts cannot legally be loaned out. A few years ago I remember seeing posts that one of the online brokers (e-trade, I believe) was switching cash accounts to margin accounts without informing customers, and even after customers requested the switch back to cash accounts, the ended up as margin accounts again shortly thereafter. I've never seen this with TD Ameritrade, as I occasionally check my account status to ensure it's still listed as a cash account.

I've seen speculation over the years that placing a sell order at a price much higher than the current share price will lock up your shares and they won't be loaned, but I've never found anything to validate this other than member posts.