Unless something has changed, this is not the case. Audio is being transcoded to AAC by the iPhone as it's being sent over Bluetooth, it is not piping the audio as-is. The maximum transmission bitrate supported is 264,630 bps and the bitrate can vary based on connection strength. This is outlined in Apple's Bluetooth Accessory Design Guideline. No matter the source, audio is being transcoded to either AAC or SBC over Bluetooth. So Apple Music AAC files are being transcoded to AAC, it isn't directly passing them through. It really isn't any different from playing AAC (or high quality mp3) over something like AAC or aptX. Some lossy formats are better at handling lossy-to-lossy transcoding and the last audio test I came across for this (on hydrogenaudio) showed that AAC wasn't bad for this purpose. They didn't test aptX but rather focused on consumer accessible codecs.