This is true. After all, it was only Rick whom Jenner told at the CDC. Though surely survivors have noticed that dead people turn even when not bitten. Gareth told his crew it was okay to eat "tainted" Bob because they cooked it, and I guess he was right?
But also, the pigs have to eat something. If Negan's group is already taking a portion of your food as tribute, why use more of what you have left to feed the pigs when they like walkers just fine? The risk, though, is that Negan's group finds out – so another reason for Ezekiel to keep it secret from the wider population.
I was going to mention the cdc but I also assume that people have figured out that death = turning, regardless of bites.
Feeding them zombies instead of their own food makes sense and feeding them the zombies just to make them appear that they were grown in the kingdom makes sense too.
As far as tainted bob, they all got murdered before we could find out but they probably knew better as they have been eating people for a while at that point.