You asked whether Zephyr is the only choice you have with NXP chips and PlatformIO as tool I said no, there’s mbed-os and also Arduino available for some boards, with the Arduino core additionally exposing some NXP SDK that you can work with directly.
If you chose to do a baremetal framework, the SDK / HAL files that you want to work with have to be in the project somewhere, in lib/
or src/
, preferrable with the same compiler flags as the real SDK would use. How you do this or where the files are sourced from is up to you.