DFM deletes only 10000 events in one run, so it is possible that most of them remained even after changing the eventsPurgeInterval. The number of events deleted in one run (by default 10000) is modifiable using the "eventPurgeMaxEvents" hidden option. I would suggest you set this to a larger value using:
"dfm option set eventPurgeMaxEvents=100000"
or some such value.
I would suggest you record the number of events to see if the deletion succeeds. You can use:
dfm report view events-history | wc -l
on Linux/Solaris
or
dfm report view events-history | find /c /v "mxzyptlk"
on windows
to see the current count of events in the database.
You can check the number of events, set the option as specified, and then check the number of events again the next day to see if events are getting deleted.