When I design the interface for that user, am I performing “front end development?”
I would say yes.
I feel you're probably trying to be too "academic" in your interpretation of the skill-sets at hand here.
A front end developer is the engineer/programmer who builds the interaction layer of your application. A back end engineer handles databases, api, etc, which receives input. If the term 'front end' doesn't fit your paradigm because you're building a voice interface (*for example, as per your pictured example), you can hire a human-machine interface engineer (HMI) before making up your own title.
Hey there. This is rough but I've been thinking a lot about jobs-to-be-done style roles and alignments, esp. with regard to hiring new designers and developers. Our company is very much organized around this job/solution relationship, and I find that "front end" and "back end" don't really hold up to scrutiny nor align with our service design. Anyway! Curious to hear your thoughts.