....because the xlock binary has been stripped out.
This binary can be found in 3.2.2 and if you add it - and 5 or 7 links/libs - it will run in port5
xfce 4.16 - tested.
There could be a slack-package for this - I don't know - I just took what I had at hand.
Btw:
If you have a port 3.2.2 running, copy the xlock binary to ~/whatever and run ./xlock -help, you will be surprised by what this binary has inside. Hint: over 20 screensaver.