Marlin uses a custom platform and arduino core to support the LPC1769, which is not part of the standard PlatformIO-provided platforms. There’s discussion at e.g. Need help getting STLINK talking to/programming an SKR1.4T 3D Printer CPU - #26 by mink007 and I have a work-in-progress example at GitHub - maxgerhardt/pio-nxp-lpc1769-arduino-test: Simple project for testing Arduino support on the NXP LPC1769 chip using a custom platform..
But if your target is to compile Marlin and not create your own firmware, above is probably not relevant. Can you elaborate on
?