Why is that halfling incapable of flying across the chasm? Is it perhaps that there is a rule to that effect the players all play by?
Nothing stops the Halfling from doing so besides their physical incapability. The Halfling is still free to run and jump across the chasm and flap their arms in an attempt to fly. They'll fail to fly because they lack the physical ability to do so, and if their jump check fails to reach the other side, they fall into the chasm. Even if they lack the physical ability to succeed, they still possess the physical ability to try. The point is that's their choice to risk the consequences of their actions. Telling the Halfling that they can't attempt to fly across the chasm would be railroading their character.
Now what happens when the Druid puts on metal armor? Is something physically stopping them from doing so? What is causing them to lack the physical capability to put on the armor? Because the Sage Advice explicitly clarified the answer to that is that nothing in the game system prevents this, and nobody can point to anything in the game system that says otherwise. The Druid is 100% physically capable of putting on any armor they wish, and there are no penalties for doing so, unless they otherwise lack proficiency in its use.