Only if you choose to play like that. Once the words "Flowing Time Trait" "Haunt Shift Trick" and "Persistent Time Stop" show up, you're already outside of the realm of things a DM would actually allow, and into theoretical optimisation.
Well yes. But the hypothetical was ONE MILLION LEVEL FIVES who then took the time to KILL EACH OTHER UNTIL THEY COULD HIT LEVEL 20.
So it's a theoretical optimization puzzle.
If you're talking about killing five or six level 20 fighters, that's really not that difficult and requires little optimization.
A Druid with even poor optimization outperformed anything a Fighter, Monk, Barbarian or Ranger could bring to the table.