Depends. You can use the amp as an USB sound card connected to your PC (possibly also smartphone) and stream music from there to your other Arylic devices. Granted, this might not be the most used scenario since there are other, more convenient “wireless” scenarios.
Regarding the IR sensor: Yes, you’re right and I already did this for my DIY boombox (see here). However, I assume most users of the amp plate might not be so much into electronics and are happy to get an “out-of-the-box” solution they can just fit into their speaker cabinets. The button board plus maybe the IR extension board will help but this solution is a bit chunky and pricey. I would have preferred an extra connector where the IR sensor would sit on the board (upper right corner) and an LED-shaped IR sensor with maybe 30cm cable. This way you could place it whereever you want in the cabinet (like using a small drill hole in the front). Shouln’t have costed more that 1-2$ in production. Even a small hole in the plate with a fixed IR sensor would probably work for most people (unless the speaker is very near to a wall).
However, this is complaining about first-world problems.