Player runs on to court

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NCAA Mens

After a 30-sec timeout, team A has 5 players on the floor ready to inbound the ball. Team B has 4 players on the floor. As A1 inbounds the ball to A2, B5 still tying his shoe on the bench runs on the floor to play defense. All players on the floor were the original players before the timeout.

Official calls technical foul for illegal substitution. Coach B does not argue the T. Game goes on.

Question: Was a T for "illegal substitution" the right call? The play looked weird enough to deserve a T but did it really deserve one?

B5 ran on the court with 4 players but was not a substitute. I tried to look up this situation in the rulebook book but could not find it. Please help me clarify my mind.