The Arduino-Pico core allows programming against the SDK (currently 1.5.1, latest one) like the docs say. There is however no way to disable the compilation of the Arduino core files – alas, if you don’t use any of the Arduino functions, most should be thrown out by the linker though. For some things though, you have to go through the Arduino functions. And whether that programming is done in the Arduino IDE or in PlatformIO doesn’t matter.
Currently I don’t know of a PlatformIO platform like WizIO’s that has gives pure access to latest Pico-SDK and I’m not working on one.