A single approved antisiphon/backflow preventer DOES replace the individual ones on the valves. However, the ones on the valves are atmospheric type, while the common one has to be either a pressure or reduced pressure principal one, depending on your area's requirements.
Remove the word 'antisiphon' to be more correct, since that word is allied with the combination of shutoff valve and atmospheric vacuum breaker, known as an antisiphon valve. When you use antisiphon valves as the means of backflow prevention, you must use one antisiphon valve for each zone, because there can be no valve downstream of an atmospheric vacuum breaker. This is an important point, because an atmospheric vacuum breaker will not function if it is pressurized 24/7 (and you don't get to claim the the downstream zone valves are always in perfect working order - backflow preventers are 'self-contained' and not dependent on other devices)
The "toxic" question comes in when codes take into account the nature of the possible backflow. If the code declares the water in a sprinkler system to be toxic, then the backflow preventers required by the code will be rated to protect against toxic backflow.
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Assembly devices are not rated for toxic backflow, so they are no longer approved by many states.
There are two ways to define the dividing line on backflow, and lawn sprinkler systems can be on one side or another. If the words "toxic" and "non-toxic" are used, the sprinkler system will very likely be defined as toxic, and the code will not allow the DCVA. If, instead, the words are "high-hazard" and "low-hazard", then the lawn sprinkler system may be classified as low-hazard, and the DCVA can be used.
If someone asks about lawn sprinkler backflow, I would point them towards backflow that is toxic-rated, because codes get tighter, but rarely get looser, and old backflow is not likely to be grandfathered when codes get tighter. And since most posts do not state a location, I would never assume the poster could actually use the DCVA.
The least expensive single device for backflow prevention, upstream of a group of zone valves, is the Pressure Vacuum Breaker. It is toxic-rated, and must be installed so that the device is at least a foot higher than any pipe or sprinkler downstream of it.