Interesting theory, but I don't think it applies here. Look at the middle picture in the teardown. The speaker assembly is glued directly to the dimpled metal part (the dimples are on the other side you can't see). So it's highly unlikely that those dimples are there to improve the vibration characteristics. The design seems to intentionally prevent bass reflex. To match the other pictures you provided, this would be a speaker attached to a solid block with no airflow around the speaker. Not saying that this is a bad design; just that the physics of an open speaker enclosure don't seem to apply.
It's not just a "headphone jack" it's also an optical audio port. https://support.apple.com/en-us/HT202730