Yes, it may make sense 'in some senses'.You may be unfamiliar with the idea, but it's not rare, and for an application like this makes perfect sense.
I still don't really understand this argument. Why couldn't an external PSU also be in something which was 'their own box', with the design and construction of the contents being entirely of their doing? Either way, they have to buy a transformer, and also have to design the rest of the power circuitry, source the components for it, and then construct it - the only difference being 'which box' they installed it in..... It would certainly be possible to have a PSU made to do the whole lot in the external PSU - but it would probably be a custom model and a custom unit costs more than an off the shelf one as well as bringing various factors into play in terms of specifying the design and ensuring that the manufacturer meets it. As they've done it, the key bits that control supply quality (especially noise) are inside their own box.
Of course - but that doesn't stop me expressing my surprise, and my lack of familiarity with this approach!But it comes down to this : whatever any of us think about it, it's how they decided to do it, and that's all that matters !
Indeed, as I wrote yesterday, 'common' though that approach might be, at least for some manufacturers, there are clearly plenty of mixers out there (many of which produce a 48V 'phantom power' supply) which are powered with just (one polarity of) 12V DC from a (presumably small and light) external PSU.
Indeed, that's much the same as I recently wrote, when I suggested that it could be regarded as a bit OTT to be talking about car batteries and mains inverters to power a little bit of electronic circuitry that does not need much more than 10W!!... It might even be practical to power it (for short periods) with 4off PP9 batteries to give (nominally) ±18V.
Kind Regards, John