@jeremy_k Alright! d) says that if you provide your software as a download, then the download of the source (Qt's source in this specific case) can be hosted somewhere else. Good to know!
b) is the case with the written offer. But a) is the most relevant: It says that if you ship the object code (i.e. the application) in a physical product (and I would count an embedded device to be such a product) you need to ship the source code on a physical medium as well. Thus, download only wont cut it. Where I was mistaken in the details is that the physical medium does not have to be the same. But it has to be a "durable physical medium customarily used for software interchange". So, this means the Qt source does not have to be on the embedded device itself (or rather can't be as it is not "customarily used for software interchange"). A CD or DVD would certainly work. I would hope that a USB thumb drive or an SD card would to the trick as well.