Fire hardened wood was still used fairly commonly by some cultures among lower warriors, more particularly among those for who iron was expensive (Germans for example) or not needed in their day-to-day work for a weapon. It's quite appropriate for poor Lusitanian herdsmen, who'd probably not have a clutch of javelins or the like. Fire-hardened javelins would be cheap, quicker to make, and more reasonably part of their war gear than iron-tipped javelins.
Edit; Aymar beat me to it. But I'll add this (in a second edit no less); compare it to the levies of Celts. Celts guaranteed able-bodied adults a spear and often a shield of some sort in every village and farmstead, and some places gave them darts or javelins. They were incredibly rich in the ancient world and had a massive surplus of iron. It was, as such, easy for them to have iron headed weapons for every combatant. The same couldn't be said of many other places, even those with good iron supply, but lacking any guarantee that there would be weapons for everyone.
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