For those wondering. This substance is regulated by the Convention on Certain Conventional Weapons (CCW), third protocol.

One attack on the town of Dhayra on 16 October must be investigated as a war crime because it was an indiscriminate attack that injured at least nine civilians and damaged civilian objects

The highlight is mine but points out their legal objection here. One may refer to Article II of Protocol III, sections 2 and 3.

• It is prohibited in all circumstances to make any military objective located within a concentration of civilians the object of attack by air-delivered incendiary weapons.
• It is further prohibited to make any military objective located within a concentration of civilians the object of attack by means of incendiary weapons other than air-delivered incendiary weapons, except when such military objective is clearly separated from the concentration of civilians and all feasible precautions are taken with a view to limiting the incendiary effects to the military objective and to avoiding, and in any event to minimizing, incidental loss of civilian life, injury to civilians and damage to civilian objects.

Per Article II section 3, it would be illegal for an indiscriminate attack that did not take all feasible precautions to limit the incendiary effect to military objectives.

As for anyone wondering, the use of white phosphorus is not a violation per Article I (1)(b)(i).

(b) Incendiary weapons do not include
(i) Munitions which may have incidental incendiary effects, such as illuminants, tracers, smoke or signalling systems

Of which white phosphorus falls into when it is not used solely for it's incendiary effects. Again, that is if Israel was justifiably using the substance.

So all of this is to say, that while Amnesty International does indeed bring up a valid point. The international law gives enough wiggle room for Israel to avoid consequences.