Hard disagree because it's all intent and how you are using it.
You mention the reason you are avoiding Milestone is because your problem player would show up late without feeling a penalty.
So you are moving from a system where everyone progresses in lockstep to one where a player exhibiting behavior you don't like will progress slower.
Lie to yourself if you'd like, but "if you do this behavior I dislike your character will fall behind" objectively is being used as a punishment. Regardless if XP can be use in a non-punishment way, the specifics of what you laid out is absolutely punishing them.
Question, you said: "Experience points mean that I as the GM don't need to consider doing anything to punish "Steve" & nobody needs to feel slighted."
If you told your players, including "Steve", I switched to XP because I don't want those who show up late to keep up with the rest of the party, do you think they would be slighted knowing your actual intent? I think they might.