I did some digging and does apple really adds proprietary DSP, or is this another fine example of apple BS marketing (?) AAC is just another codec, it's not magic.
The context of what I was saying is also rather important to arrive at the answer to your question. It is not proprietary DSP, but Apple is a locked system (so in a way it is, but only because you can't change it). And it isn't necessarily in the codec that the DSP is being done, but through the music player (and Apple ecosystem) itself. AAC does not do any DSP. It is simply a way of compressing file size with as little loss to the original sound as is possible (just like .mp3 is). You are right, it is not magic, though is a very good lossy compression, and also a good (lossy) BT codec (I'm sure you know these two things are different).
On the one hand (in android) you can set a player to "bit perfect" (UAPP or Neutron as example) and if you try and use EQ, it will not affect the sound. This means that the sound is not being altered (DSP) before being sent to the head gear. This same experiment can be done in any iDevice and you will find that you can affect the sound with EQ at any time.
Now, let's take an example where you are using your iDevice and android phone out to an external DAC/amp. You would expect that, if the music player were bit perfect, you would not hear any difference in sound if switching between an android transport and an iDevice because at that point the music player is simply a transport (taking the digital, not analog, signal out to the external DAC/amp). All the DSP SHOULD be happening in the external amp. The point is that if you change from android to iDevice and back again to the same external DAC/amp and head gear you should hear no difference unless DSP is being applied before being transported out to the external DAC/amp. There is no way to avoid this in any iDevice (AFAIK), unless you Jailbreak the iDevice. Android also has a sort of limitation, but only after reaching above 24/48 where it will resample (which is DSP) and will make it sound different than the original. Neutron and UAPP bypass this by using their own "high resolution" driver and bypassing android's.
Where this applies to TWS is that the iDevice (or android) is the transport and the TWS device is the combination of the DAC/amp and head gear from the example from above. If the digital file is being transported with no DSP, then it should sound exactly the same whether it comes from android or iDevice.
Now, where this can be different between android and iDevice is in the implementation of the codecs. But, this still doesn't necessarily "color" the sound, it simply might sound different (less/more high quality) because of the (different) data that is thrown away (I know we're nitpicking here, but there is a difference between DSP and lost data from lossy codecs). So, let's pretend that the implementation of AAC is the same on both the android and iDevice. You will still hear a difference because the Apple ecosystem does do DSP (with no EQ, their version of flat, is not actually flat). Forcing a flat EQ is still DSP, where no EQ at all is just what it sounds like; no DSP (if that makes any sense).
Having said all of that, this is not always a bad thing, nor am I picking on Apple. For many, it is a desirable DSP, and in most cases, it matters not at all. But for those seeking to be as close as possible to the original this could be an issue that they have no control over (it is rather minute in the grand scheme of things). Besides TWS is still not to the point of being completely transparent in the sound (though a lot closer than some years ago), so it really doesn't matter in the end. I was merely throwing my thoughts out there as more of an observation of (possible) interest.
I still feel I might get the APP2 eventually, but they are crazy expensive here. 
