PlatformIO disables by default the Telnet server of the OpenOCD instance, unless you set e.g. debug_port = 127.0.0.1:3333 in the platformio.ini and start OpenOCD on the commandline manually.
However, you should be able to equivalentely trigger the command by just using “monitor <original openocd command>” as an extra GDB command (which GDB will forward to OpenOCD).
https://docs.platformio.org/en/stable/projectconf/sections/env/options/debug/debug_extra_cmds.html