That’s correct, the main reason the ADS1115 is on the Mayfly is to give you much better resolution on analog measurements (16-bits versus the ATmega1284’s 10-bit ADC). It can also handle a much faster sampling rate and can take differential measurements. The standard ATmega analog inputs are still available for you to use, they’re just only accessible through the 2×10 header socket. The ADS1115 pins are available in two places: the 2×10 header socket and the 2 Grove sockets. Since they’re electrically the same pins, you can’t use more than one of those locations at a time. In your case, the Grove sockets are most convenient, but for anyone using a protoshield or building their own interface board for the Mayfly, then using the headers is more convenient.