Open it, and your reasoning for why is correct. A rattle after an impact is worth finding rather than living with.
The most common things that come loose in this situation, roughly in order. A screw backing out of its boss, which is the one you are right to worry about, because a loose metal screw moving around inside a powered device can bridge contacts. A rumble motor's eccentric weight coming off its shaft, which rattles loudly and would also make that motor feel weak, so worth checking in a vibration test. A broken internal clip or a fragment of plastic from a boss that cracked on impact. And occasionally the battery contact assembly shifting in its seat.
The uneven flex you measured supports the last two. If a clip has snapped or a boss has cracked, that side of the shell is no longer held properly, and the tension test picks that up even though nothing has failed functionally. That is a genuinely useful corroboration from two independent measurements, and it is why I would not dismiss this as a harmless rattle.
The practical risk is not immediate. The controller works, so nothing is shorted today. But a loose screw rattling around during play is the one item on that list that can cause real damage, and the cost of looking is twenty minutes and no parts.
When you open it, work over a tray so nothing escapes, note where everything sits before you disturb it, and check: every screw boss for cracks, both rumble motor weights still on their shafts, all edge clips intact, and nothing loose in the shell halves. If you find a cracked boss, the controller will still assemble and hold with the remaining screws, but reassemble clips-first and tighten in a cross pattern so the shell closes without tension.
If you find nothing at all and the rattle persists, check the triggers: a trigger return spring that has come off its post rattles and would also show in a spring rebound test.
