With these new Macs, there are actually two different output devices. The headphone jack and internal speakers are separate devices, completely independent.
This change means that it is possible to send different audio sources to each output. I asked friends and colleagues to test several other newer Macs. It appears that different output devices are also present in both MacBooks Pro Apple released in July this year and iMacs Pro, which started shipping at the end of 2017.
Update (2018-11-16): Rob Mathers :
The ability to swap audio outputs while pairing a pair of headphones is something I've thought Macs should have since I got my first MacBook Pro.
Stay tuned by subscribing to the RSS feed for this post.