What this calculates
Sun and Moon positions are computed from standard astronomical series and converted to the sidereal zodiac using the Lahiri ayanamsa — the Indian government's official standard. The Sun is accurate to about 0.01°, the Moon to about 0.1°, which is well inside a 13°20' nakshatra except within roughly half a degree of a boundary, where the result is flagged as uncertain.
It does not produce a full natal chart. Positions of the other planets, house cusps and dasha periods need a full ephemeris, which this app does not carry — and no calculation is being passed off as a prediction.