Yes - so this is a common misconception. Boundaries can absolutely be upheld without punishment. The thing is if he's hitting someone, you're not standing there merrily letting him hit saying "Okay, you've lost 5 minutes screen time.... Now ten.... Now fifteen" as he's hitting his sibling multiple times are you? Your immediate reaction would be to stop the hitting by restricting his arms, moving him somewhere else or moving the sibling to a place of safety or placing yourself in between them for example.
That's the boundary. You've held it. The punishment you add on afterwards is something that you do to (in theory) help reinforce the message that hitting is not tolerated and if he does it then it will have a negative consequence for him, with the aim being that this will reduce his motivation to hit and reduce or stop the hitting.
But if that doesn't work, or you just don't want to use punishment for whatever reason what do you do instead? Remember your aim is to reduce or stop the hitting. A punishment aims to do this by making it less rewarding for him. That assumes a lot of things. It assumes that the hitting is rewarding for him in the first place (maybe it is - sometimes your sibling is really annoying and hurting them seems like a reasonable goal when you're six. Maybe it makes them go away and stop touching your stuff.) It assumes that he already knows why he shouldn't hit, understand the effect on the other person. It assumes that he knows and is capable of using alternative ways to express his annoyance, get sibling to stop doing something, whatever it is.
So you could address any of these points instead of trying to adjust the motivation. Education can also help adjust the motivation btw by making an approach seem less appealing - I know that hitting my children would probably make them stop doing annoying things but I know it's not an approach I want to use for other reasons, so I don't. Albeit this might be a bit tough for a 6yo to grasp.
Or you could work on problem solving alternatives.
Or you could look into the triggers -what's happening immediately before the hitting, can you see a pattern with any markers you can step in earlier? Can you help avoid any of the situations? Is there something we can put in place? Do we have to think like having a 3yo and newborn and just never leave them alone together? Is that practical? What is practical?
When you change the goal from how to punish this behaviour to how to reduce or stop this behaviour, it really helps open up options and see things in a clearer way.
It would be exhausting to do this for every single issue you're having at any time so the CPS model recommends focusing on the top three most disruptive or biggest problems. For everything else, hold the boundary, reiterate the boundary (I'm not going to let you hit her) but don't worry about the other steps for addressing it until you've moved on to addressing that specific problem.