I did a quick local verification after building the CLI from the fork. I did the followings:

I executed the the debug command from a terminal:

/path/to/arduino-cli debug -p /dev/cu.usbmodem14602 -b arduino:samd:arduino_zero_edbg --interpreter mi2 /path/to/my/sketch -v --log-level trace

I got back the expected output:

DEBU[0000] Executing debugger                            param0=/Users/akos.kitta/Library/Arduino15/packages/arduino/tools/arm-none-eabi-gcc/7-2017q4/bin//arm-none-eabi-gdb param1="--interpreter=mi2" param2=-ex param3="set remotetimeout 5" param4=-ex param5="set pagination off" param6=-ex param7="target extended-remote | \"/Users/akos.kitta/Library/Arduino15/packages/arduino/tools/openocd/0.10.0-arduino7/bin/openocd\" -s \"/Users/akos.kitta/Library/Arduino15/packages/arduino/tools/openocd/0.10.0-arduino7/share/openocd/scripts/\" --file \"/Users/akos.kitta/Library/Arduino15/packages/arduino/hardware/samd/1.8.8/variants/arduino_zero/openocd_scripts/arduino_zero.cfg\" -c \"gdb_port pipe\" -c \"telnet_port 0\"" param8=/Users/akos.kitta/Documents/Arduino/sketch_aug24c_x1/build/arduino.samd.arduino_zero_edbg/sketch_aug24c_x1.ino.elf
=thread-group-added,id="i1"
~"GNU gdb (GNU Tools for Arm Embedded Processors 7-2017-q4-major) 8.0.50.20171128-git\n"
~"Copyright (C) 2017 Free Software Foundation, Inc.\n"
~"License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>\nThis is free software: you are free to change and redistribute it.\nThere is NO WARRANTY, to the extent permitted by law.  Type \"show copying\"\nand \"show warranty\" for details.\n"
~"This GDB was configured as \"--host=x86_64-apple-darwin10 --target=arm-none-eabi\".\nType \"show configuration\" for configuration details."
~"\nFor bug reporting instructions, please see:\n"
~"<http://www.gnu.org/software/gdb/bugs/>.\n"
~"Find the GDB manual and other documentation resources online at:\n<http://www.gnu.org/software/gdb/documentation/>.\n"
~"For help, type \"help\".\n"
~"Type \"apropos word\" to search for commands related to \"word\"...\n"
~"Reading symbols from /Users/akos.kitta/Documents/Arduino/sketch_aug24c_x1/build/arduino.samd.arduino_zero_edbg/sketch_aug24c_x1.ino.elf..."
~"done.\n"
=cmd-param-changed,param="remotetimeout",value="5"
=cmd-param-changed,param="pagination",value="off"
~"Remote debugging using | \"/Users/akos.kitta/Library/Arduino15/packages/arduino/tools/openocd/0.10.0-arduino7/bin/openocd\" -s \"/Users/akos.kitta/Library/Arduino15/packages/arduino/tools/openocd/0.10.0-arduino7/share/openocd/scripts/\" --file \"/Users/akos.kitta/Library/Arduino15/packages/arduino/hardware/samd/1.8.8/variants/arduino_zero/openocd_scripts/arduino_zero.cfg\" -c \"gdb_port pipe\" -c \"telnet_port 0\"\n"
&"Open On-Chip Debugger 0.10.0+dev-gf0767a31 (2018-06-11-13:48)\n"
&"Licensed under GNU GPL v2\n"
&"For bug reports, read\n"
&"\thttp://openocd.org/doc/doxygen/bugs.html\n"
&"Info : auto-selecting first available session transport \"swd\". To override use 'transport select <transport>'.\n"
&"none separate\n"
&"adapter speed: 400 kHz\n"
&"cortex_m reset_config sysresetreq\n"
&"Info : Listening on port 6666 for tcl connections\n"
&"Info : Listening on port 56621 for telnet connections\n"
&"Info : CMSIS-DAP: SWD  Supported\n"
&"Info : CMSIS-DAP: Interface Initialised (SWD)\n"
&"Info : CMSIS-DAP: FW Version = 02.01.0157\n"
&"Info : SWCLK/TCK = 1 SWDIO/TMS = 1 TDI = 1 TDO = 1 nTRST = 0 nRESET = 1\n"
&"Info : CMSIS-DAP: Interface ready\n"
&"Info : clock speed 400 kHz\n"
&"Info : SWD DPIDR 0x0bc11477\n"
&"Info : at91samd21g18.cpu: hardware has 4 breakpoints, 2 watchpoints\n"
&"Info : accepting 'gdb' connection from pipe\n"
&"target halted due to debug-request, current mode: Thread \n"
&"xPSR: 0x41000000 pc: 0x000022cc msp: 0x20007fcc\n"
&"Info : SAMD MCU: SAMD21G18A (256KB Flash, 32KB RAM)\n"
=thread-group-started,id="i1",pid="42000"
=thread-created,id="1",group-id="i1"
~"resetExternalChip () at /Users/akos.kitta/Library/Arduino15/packages/arduino/hardware/samd/1.8.7/cores/arduino/Reset.h:29\n"
&"29\t/Users/akos.kitta/Library/Arduino15/packages/arduino/hardware/samd/1.8.7/cores/arduino/Reset.h: No such file or directory.\n"
*stopped,frame={addr="0x000022cc",func="resetExternalChip",args=[],file="/Users/akos.kitta/Library/Arduino15/packages/arduino/hardware/samd/1.8.7/cores/arduino/Reset.h",fullname="/Users/akos.kitta/Library/Arduino15/packages/arduino/hardware/samd/1.8.7/cores/arduino/Reset.h",line="29"},thread-id="1",stopped-threads="all"

I also dropped the CLI into the 0.1.0 Pro IDE and could start the debugger.