That is certainly true, but it does not mean those products are nitrate-free.
Nitrates are naturally occurring in many vegetables and can not be removed.
That said, since nitrate levels in vegetables can differ depending on growing conditions (i.e.: the amount of nitrogen in the soil and water), it is very possible to get batches of spinach with higher amount of nitrates than others.
When screening nitrate amounts, individual companies simply set up their own criteria ("we only use vegetables containing x number of parts per million of nitrates or less") and apply it when making baby food.
"Screened for nitrates" does not mean "nitrate free."