In the specific context of setting the adb tcp/ip port, the adb tcpip 5555 command does restart the adbd service.
As one can see the PID of adbd is changed after the adb tcpip command :
$ adb shell ps | egrep -w "adbd|PID"
USER PID PPID VSIZE RSS WCHAN PC NAME
shell 3675 1 4720 300 ffffffff 00000000 S /sbin/adbd
$ adb ...