In January 2015, Tillie was highlighted by ESPN among five of the best international players in the 2015 and 2016 classes with aspirations to play college basketball.

He was drawing major interest from California at the time, and scouts were impressed with his terrific offensive rebounding, athleticism and high motor.

[6] In August 2015, Tillie traveled to the United States and took official visits to Georgia Tech, Utah, and Gonzaga.

[4] Sources: He made his first appearance in a Gonzaga jersey on November 5, 2016, when scoring four points to go along with four rebounds, three assists, two blocked shots and two steals in twelve minutes of play against West Georgia.

[9] He saw 14 minutes of action in the NCAA championship game against North Carolina, pulling down nine rebounds and scoring one point.

[10] As a sophomore in 2017–18, Tillie was Gonzaga's second leading scorer, averaging 12.9 points in 26.2 minutes per game to go along with 5.9 rebounds and 1 blocked shot per contest.

[23] Tillie helped the French Under-16 National Team win the 2014 FIBA Europe Under-16 Championship, averaging 14.3 points, 9.6 rebounds, 1.7 assists, 1.2 blocked shots and 1.2 steals a contest, while being recognized as the tournament's MVP.

His team defeated USA, featuring fellow Gonzaga commit Zach Collins, in the quarter-finals, before losing to Argentina in the semifinals, and thumping Spain in the third-place game.