FWIW this bit in the article is a little confused. The SIMD instructions being detailed appear to be Espressif-custom things implemented using Cadence's "TIE" facility.
Cadence does indeed have their own SIMD architecture ("HiFi", really it's a family of similar but binary-incompatible ISAs). And indeed docs for that don't appear in public (though if you look carefully, details for how to emit the instructions are part of the GNU toolchain integration).
But that isn't this. If you want docs for this, talk to Espressif, not Cadence.
Cadence does indeed have their own SIMD architecture ("HiFi", really it's a family of similar but binary-incompatible ISAs). And indeed docs for that don't appear in public (though if you look carefully, details for how to emit the instructions are part of the GNU toolchain integration).
But that isn't this. If you want docs for this, talk to Espressif, not Cadence.