Your win rate is good on the maps you mastered but that does not qualify you as a smurf because you have a normal play rate by game duration. Map dodgers have a disproportionately large chunk of their games be short and lost, see this. Non-smurf people who are good on one map will always give back their points on other maps, it’s not different from playing only one top civ. That’s not a contradiction with the need to stop obvious rating-droppers and map-dodgers.
Also, you say you have 70-80% on Arena/Islands and 20% on Nomad but you have 144 / 6 / 9 games on those maps so don’t worry too much about a win rate for a map you barely played.