Is it? I guess anyone that read the RFC or saw one implementation can easily reproduce it from the top of their head. Of course some details can be fuzzy and it's not that easy under stress but base64 is definitely not rocket science.
As with most coding questions, the important parts are when you start looking at corner or edge cases. Or when you refine your initial implementation.
Pretty simple, easy to understand question. I wouldnt consider this hardcore - but over the phone it doesnt make much sense to ask such questions. Hope there was at least a Zoom or Skype meeting or something.