Can Vex, Apathetic's ability stun Baron Nashor when Baron Nashor is played, given that Baron Nashor has 'I can't be chosen by enemy spells and abilities'?

Ruling: Yes, Vex, Apathetic's ability will stun Baron Nashor when it is played. Baron Nashor's protection from being chosen by enemy abilities does not apply because Vex's ability programmatically selects the unit rather than having its controller choose a target.

Sequence:

• Baron Nashor is played by the opponent
• Vex, Apathetic's triggered ability activates
• The ability programmatically selects the unit that was just played (Baron Nashor)
• Because the unit is programmatically selected rather than chosen, it is not considered a target
• Baron Nashor's protection does not apply to non-targeting effects
• Baron Nashor is stunned and cannot move that turn

Nuances:

• The distinction between "programmatically selected" and "chosen" is critical - when an ability automatically identifies a game object based on what triggered it (rather than requiring a player to choose), it is not targeting
• Baron Nashor's protection only prevents it from being chosen, not from being affected by programmatic selection