OK… man pgrep. And there’s a line towards end of file: “The process name used for matching is limited to the 15 characters present in the output of /proc/pid/stat. Use the -f option to match against the complete command line, /proc/pid/cmdline.”
So the name xfce4-taskmanager is too long for pgrep to find unless used with the option -f. Same option finds Cadence as well, which turns out to be a Python process.
Note to self: RTFM.