Any wire (of a large enough guage) can be used for wiring irrigation valves (and 16AWG should be plenty big enough unless the controller is going to be more than something like 500' away from the valves). The biggest question is will is hold up directly burring it in the ground.
The "GASOLINE and OIL RESISTANT" wording seems to indicate the sheathing should hold up in the ground. Beyond that, you might want to judge based on the thickness of the sheathing. Does it seem stout enough that it should avoid getting cut from simple abrasions with rocks as it gets installed?
The next question is that the TFFN sounds like something to do with conduit, which makes me think you have individual strans of wire rather than a bunddle of wire. If that is the case, I would think the individual wires would be more prone to damage than if the wires were in a bundle. If they are individual wires, you might want to think about putting them inside conduit for a little added protection.