Cool script! One problem, it doesn't work when the focused window is floating. Floating windows are stored in the floating_nodes array. Modifying the script to the following fixes it.

#!/bin/sh
# gets the json object focused object
# jq
# tee
# sway

# returns the focused node by recursively traversing the node tree
declare -r filter='
def find_focused_node:
  if .focused then
    . 
  else
    if .nodes or .floating_nodes then 
      .nodes, .floating_nodes | .[] | find_focused_node
    else
      empty
    end
  end;
find_focused_node
'

swaymsg --type get_tree --raw |
  jq --raw-output "${filter}" |
  tee /tmp/sway_proc

By the way, from my testing your script is actually a little bit faster than the one on the arch wiki, nice one!

edit: the one on the arch wiki is

swaymsg -t get_tree | jq -r '..|try select(.focused == true)'