In this illustration, everything is protected from not just back siphoning, but the positive water pressure that is the result of being below wl. As others have pointed out though, anti siphons can become inoperable from a clogged vent. But they're better protection than having everything, pumps, fittings, hoses and bladder or tank, in the potential inflow of below wl pressure.

Without a siphon break, any connection in the system that fails, including the tank, hand pump, valves, fittings and hose connections, could sink the boat while you're on shore.

With the siphons, assuming they work, the only potential for sinking is the short length of hose to each siphon and, it's connection to the seacock(s)

The only sure things are closed seacocks(assuming they don't fail....).

The only thing wrong with this illustration is the loop before the WC isn't between the pump and bowl. Oh! And the tank isn't vented.