That is exactly what the person you're replying to meant.
My understanding is that if you had $100 and you purchased stock for $100 and it went up in few hours to $120 and you decided to sell it, so you can purchase something else. You can't you have to wait 2 days.
Of course if you have enough money you will have buffer to account for that, but it makes it harder to do day trading when everything is delayed by 2 days.
The "you can't part" here is what's not clear. My understanding of non-margin trading is that you would be able to sell for $120. What you would not be able to do is then purchase something else with that $120 until T+2 when it settles and the money is in your account again. You technically don't have that $120 until the settlement.
If you can't sell the stock that you bought on the same day that you bought it, then by definition you can not day trade.
Deposit $1000, wait 3 days, buy and sell $100 of the same stock in the same day. You still have $900 to trade with.
It may be “less efficient”, but it also reduces your risk, and risk management is fundamentally what successful trading is about.
Some people don’t want to depend on the bank for margin or leverage, and are happy to trade with cash only. Increased efficiency brings increased risk and decreased ability to deal with short term shocks.