Flashmagic also works with .bin files. The PlatformIO Marlin build will output that .bin file (in .pio\build\<environment>\firmware.bin), which you may attempt to flash. See my text in SKR 1.4 Turbo Motherboard Without SD-CARD Reader! Bricked? - #4 by maxgerhardt regarding the bootloader and possible offset address which might not make this work.