To be clear, we're talking about the fitness coach, not the physio. See below.
You seem to be ignoring the facts of what has already happened. Sinner told them what he says happened. Ferrara, the fitness coach, brought the cream in his kit. He gave it to the physio for his cut hand. He says he told him not to go anywhere near Sinner with it. But the physio gave Jannik a massage without gloves on. This is their story, and WADA and the ITF found it plausible. The fine (?), the short ban and the loss of money, points was his punishment, because, by the rules, he is ultimately responsible for his team. I don't think you can call it an "excuse" if it's the truth. It's an explanation of what happened. Of all of them, it was the physio who is the most to blame, if you buy the story. He hasn't been re-hired. As I said, I think Ferrara is guilty of an error in judgment for bringing it in the first place. I think Kieran's argument is that it precipitated a situation that make tennis look bad, which makes it a higher crime. I can understand that.
However, if you're talking in the theoretical future, which we kind of were, then, yes, I agree that you can't just punish the team member and absolve the player. But you are still talking about this present case. Sinner was held ultimately accountable. Now, you can disagree with the punishment, and you're welcome not to buy Sinner's story, but it's a closed case. It will be interesting to see how fans on social media react to it, and if he gets a lot of negative blow-back. If he wants people to forget about the drug ban and move on, this was perhaps not the wisest choice.