[reso]

I believe the speaker, when saying that physicists have stopped asking "what is matter", means that physicists have focused themselves on questions about the rules that govern matter and energy, and not on the question of what meaning these concepts have.

If we imagine nature as a board game to which we don't know the rules, but can see some of the set of pieces set before us and observe their interactions, physics focuses on the question "what are the rules to this game?" Our speaker in this question is asking "what are these pieces made of?"

String theory, as an example, does not posit that particles are made out of strings, it posits that we can model particles mathematically with a structure that we call a "string". In this use of the word, "string" is a metaphor. What the nature of a "string" is--whether it is a coded abstraction in a computer simulation, whether the math is ground truth itself, whether this question is intractable to the human mind--is left up to philosophy.