Valid debug_tool values are only that listed in the documentation plus custom. PlatformIO does currently not have built-in support for the Atmel-ICE debugging probe, it may however achieviably manually by avarice and the appropriate debug_server configuration directives amongst others, as indicated in Debugging AVR / Arduino with ATMELICE / JTAGICE3 and avarice / avr-gdb and CLion / Avarice debug not working.