That is not my understanding of the rule. It has to do with the same sport. If you are playing Basketball and there is a Open Gym for baseball it does not do anything to the players’ eligibility. It has to be something with the coach not wanting his player to be hurt during the open gym.
Here is the rule from the IHSA:
3.101 During the school season for a given sport, in a school which maintains a school team in that sport, a student shall not
participate on any non-school team, nor as an individual unattached in non-school competition, in that given sport or in
any competition that involves the skill of the sport in question. Violation shall cause ineligibility for a period not to
exceed 365 days. An exception may be made by the Executive Director under the guidelines adopted by the Board of
Directors for competitions sponsored and conducted directly by the National Governing Body for the sport.
3.102 Students may participate in a tryout for a non-school athletic team while a member of a high school team in the same
sport, provided the tryout is exclusively a demonstration of skills with no practice or instruction involved. Astudent shall
be considered to be a member of a school team when he/she engages in any team activity, including but not limited to
tryouts, drills, physical practice sessions, team meetings, playing in a contest, etc., on or after the date specified in Bylaw
5.000 and its sub-sections