I think I actually just fixed this. Now I feel stupid for not looking at this more closely - the error being thrown is "line 1" - which I assumed was something in the script being executed, but it was really just saying "hey, this file isn't executable" - .js files typically aren't, but that's how this workflow is setup - and now, apparently in Sonoma, those files need to be executable. So I ran
$ chmod +x ~/Library/"Application Support"/Alfred/Alfred.alfredpreferences/workflows/<my GUID for this workflow>/*.js
Now everything appears to work.
❯ ls -l ~/Library/"Application Support"/Alfred/Alfred.alfredpreferences/workflows/user.workflow.D2D2FD7C-8951-4DFB-9011-F215D92C0F9E/"
total 1208
-rw------- 1 bruth staff 190371 Aug 27 08:34 05291F67-A2F4-49A2-A3AA-23923B919638.png
-rw------- 1 bruth staff 127297 Aug 27 08:34 1EF5C902-0F47-47F1-BCBB-5BA273DE5101.png
-rw------- 1 bruth staff 34514 Aug 27 08:34 45BA3C52-3F5F-462D-8F18-DE516A900E69.png
-rw------- 1 bruth staff 81857 Aug 27 08:34 7E9CA23F-2FF6-47EF-993D-6157788B9FD4.png
-rw------- 1 bruth staff 85450 Aug 27 08:34 C670C61E-A7A8-4A02-9A61-453AAD409775.png
-rwx--x--x 1 bruth staff 686 Aug 27 08:34 focus-tab-webkit.js
-rwx--x--x 1 bruth staff 463 Aug 27 08:34 focus-tab.js
-rw------- 1 bruth staff 37446 Aug 27 08:34 icon.png
-rw------- 1 bruth staff 28878 Aug 27 08:34 info.plist
-rwx--x--x 1 bruth staff 1248 Aug 27 08:34 list-tabs-webkit.js
-rwx--x--x 1 bruth staff 1313 Aug 27 08:34 list-tabs.js