I agree you should have been told, so they've dropped the ball there, but I don't agree further action should be taken (assuming this is a one off and this child doesn't go round 'strangling' other children every day?)
They lost their playtime and apologies were issued. I would hope they've had it explained how dangerous it is and never to do it again. What further action would you expect? They're 6. At that age a reaction is a reaction. Kids don't grade 'violence' like we do. Things like biting, kicking, headbutting, punching etc will always have a worse rep with adults than a hit or push, and strangling sounds especially alarming, but children are still learning about boundaries and acceptable behaviour. As long as they learn from such outbursts that it is completely unacceptable, that's all we can ask. We can't expect them to automatically know how dangerous such an act is, until it's explained to them. Further punishment for something they perhaps didn't know was dangerous would simply be punishing them for being young and naive. If it happens again, after it's been explained to them, that's different. As a first offense, I think a missed playtime and apology is sufficient at age 6.
To be honest, 'strangling' sounds horrific but, in reality, it was most likely a grab around the neck. Dangerous, inappropriate, but easily exaggerated. As the school didn't even think to tell you, I"d hope this is the case here. A minor incident in reality which sounds much worse in how it has been described to you by your 6 year old and his friend. If I had a pound for every child who claims to have been 'punched', when in reality they were hit, open palm, I'd be very rich. Not nice, but not quite how reported by a 6 year old. Their limited life experience and vocabulary often means things get lost in translation.
I would ask school for the facts but be prepared for the fact it's likely case closed as far as further action is concerned.
Ultimately, your child shouldn't be pushing kids around any more than this other kid shouldn't be putting their hands around someone's neck. Remind him of that as, although the other child's actions were wrong, he's not completely innocent here.