digital strength

As you rightly pointed out, Moon does not have digital strength on the 3rd day after Amavasya. It means one source of strength attributable to Moon is not there. However, to know the real strength, one shall calculate it collectively. 

In the present case, though Moon didn't have digital strength, it has sthana bala being placed in exaltation sign, conjunct with beneficial Jupiter.

Your point was how bahula tritiya was accepted for muhurta.

Point here, I think is that Moon came out of asthangata being more than 30 degrees aeay from Sun. The Moon is waxing Moon and always auspicious, though it has a bit low on digital (paksha) strength. But Moon has strong positional (sthana) strength, and Jupiter's samagama in Taurus in GajaKesari yoga enhances the combination strength multifolds.