To my knowledge, this is Congress asking questions, not sending a subpeona. Participation at this stage is still voluntary, and mostly consists of political posturing.

If Congress wishes to pursue it, they have legal tools where cooperation is non-optional. The current conversation is part of the song-and-dance where Apple tries to avoid that by offering something resembling cooperation, but on their own terms. Many of the questions are meant to answer the question "if we conduct a probe for realsies, who will be the winner?" rather than the question-as-asked.