Espressif is all-in on RISC-V, expanding their portfolio of RISC-V devices where they previously had only XTensa: ESP32-S31 is the first big departure from the coincidental alignment of ISAs within their product structure and definitively ends further debate about what those letter designations mean.

BTW, S3 has an RISC-V core in addition to the XTensa cores. That's the part that's running in deep sleep.

In practice, most Espressif users barely know or care what ISA is in play: they have ESP-IDF and the Espressif libraries papering over the difference for nearly all purposes.