No one goes to the national finals with just one player, so we know that Indiana State had a good team in 1979 - more than just Larry Bird - even if they were not from a power conference. Bird did have a double/double, but there were two other players who scored in double figures.
And even though that was their only run as far as the Sweet 16 at the D1 level, they did have considerable success at lower levels.
The Sycamores were the NAIA national champs in 1950, and the runner-up on two other occasions, plus the D2 runner-up in 1968. And they went to the NAIA Final Four on three other occasions. So, Indiana State was accustomed to the bright lights of post-season, although it had been a while when the Bird Man led them to within a few points of a national championship in 1979.