On some other boards the voltage regulator can be disabled as a feature implemented by design (by solder jumper or external pin) to prevent it drawing quiescent current when the board is externally powered with 3.3V.
Disabling the regulator apparently also prevents it drawing quiescent current.
If this works the same for all regulators with EN pin I don’t know but it’s possible to give it a try.
(I’m not sure if all regulators can handle 3.3V external input on their output pin when they are disabled.)
Another part that will probably negatively impact sleep current of the board is the battery charger chip.
If you want to power the board by battery directly on the 3.3V pin without quiescent current losses from a voltage regulator then it’s possible to use LiFePO4 (lithium ion phosphate) cells which have a nominal voltage of 3.2V. (Be aware that these cells usually don’t have an overdischarge protection built in.)
When powering a simple board externally with 3.3V (e.g. Arduino Pro Mini) it often helps to remove the voltage regulator from the board to minimize sleep current. For more advanced boards that include a battery charger removing only the voltage regulator will be much less effective (and more difficult because of many smaller SMD parts).