I've swiped my child, for clarity, not hit, not smacked, but I have swiped my hand up to move her hands and that has pushed her backwards onto the bed.
My DD was going through a big phase of hitting, it was the middle of the night and she wasn't happy it wasnt time to get up - she started hitting me, mid asleep so I just swiped upwards (not hitting, but making contact on my arm to her arms)
I think it also does depend on tone and after the event. In these situations, I talk to DD (4) most of the time and try to de-escalate, find other ways to get frustrations out (I found non-aggressive stamps with feet, or hard jumps) as good way of redirecting it, she the learnt about breathing and most of the time she understands the emotions now. We always debrief afterwards, why it happened, how can I be prevented in the future, how we can do differently next time, we always apologise and say 'friends again' and state we love each other.
I came from an abusive household, there is a difference between asserting your boundaries and respect for not being hurt and disrespected, when you do are not showed it from them.
I think it is age developmental, and ife wasn't always this calm and easy. We have had times like this and frustration = frustration, but the more you deal with it, it is likely to happen.
The walking out and being huffy all day, does sound as though Dad is struggling somewhere. He needs positivity and support also, this is new to him, you and your DC.
Happy to advise any further if I can