it appears as if gpp already features the std::abs() for fp since C14 or even 11, opposite to C++ standards only since 17
Compiler Explorer
std::abs(float), std::fabs, std::fabsf, std::fabsl - cppreference.com
float computations (fp32, fp64) evaluate differently vs. ARM Cortex M0+M3+M4 & Mega2560 (IDFGH-1084) · Issue #3405 · espressif/esp-idf · GitHub
Thanks for the links regarding the GCC and C++.
Yes, it appears to be a complex mix of Arduino, compiler and C/C++ standards. Thanks for flagging this up on Github. At least situation is now sorted, or a least will be once ESP32's "Arduino.h" is officially updated.