Really just curious :)
When working with embedded devices like that, the main thing is you can't just easily import libraries or use many layers of abstraction (limited stack depth too). So generally everything is done at a low level.
While technically the customer asked for the solution to problem A, they wanted us to solve problem B, but we failed to invoke our inner patio11, and as a result delivered what they said instead of what they meant. A frustrating lesson for me. At that point in my career I should have known better already.
ETA: sibling comment about Cortex-M0 is spot on. You really start counting bytes. Some kiB of RAM sound a lot, but when you have to juggle several tasks, and don't have to option to write from scratch but instead reuse existing old code you've lying around, stuff gets dirty pretty quickly. Other team member was doing the hard work, I was just providing tooling for them.