How long ago was your DNephew's diagnosis @IGrewUpInTheFallOut, because - and I mean this as gently as possible - it doesn't sound as if you have much of an idea about what being on the Autistic spectrum actually means, especially in relation to your DN? You acknowledge that his behaviour has quite recently improved a lot, so presumably you recognised before that, that he was not very good at self-regulating himself? But then young NT children also have to learn to self-regulate, and that often isn't easy for them either.
Do you have any children yourself OP? I'm sorry if you have already shared that information with us (unfortunately - and mentioned with some embarrassment - I don't know how to leave this post to check your pp, without losing what I have already written when I try to come back 🤔🙄) After seeing your photo of the 'bite', it really doesn't look as if he ever meant to really hurt you, it hasn't broken the skin, and as you say, he is nearly as tall as you, and quite a bit stronger, so if he had intended to hurt you, he could have done so quite easily.
I think that maybe he was just really frustrated with you, as you didn't mention going up to see him about 20-30 minites before you wanted him to go to bed, in order to warn him that his bedtime was approaching, and perhaps even asking him if he would like you to bring him up a hot chocolate or something? Did his DM tell you what his bedtime routine is, maybe you didn't follow it quite closely enough? If you do love and care about your DN, and indeed your DSis too, I think that it would be a very good idea to read some books about how Autism can affect children from a very early age - even if your DN wasn't diagnosed so early - so that you could hopefully see a pattern emerging, and progressing throughout his childhood.
If you don't have children of your own, maybe reading some articles and/or books concerning the childhoods of various NT children would also help you, especially with putting their behaviours into some kind of context. Of course, it is quite unlikely that you will read anything about a child who's trajectory is almost the same as your DN's, but hopefully it would help you to understand - even a little bit - how very uncomfortable, and even traumatised, a person with a ND diagnosis - and particularly a ND child - can get. Obviously, anyone who has a ND MH condition, will be one of thousands or even millions of individuals (depending on the size of the place you are reading about, eg a small town, a city, a country, a continent, or even the world).
Therefore, if you want to get a good overall picture of all things Autistic, you may need to read many articles and books, and listen to some recorded lectures, and very importantly, get as much information as possible - through whatever medium - straight from the parents of children with Autism, and/or other ND conditions, and when possible, also from adults who have the condition themselves. One of the early truths you will discover, is that when children with Autism appear to act badly , it is almost certainly because they are in a bad/scary/sad (mental) place, these children must not be punished because of something they have little or no control over. If you don't believe anything else, please believe that punishing a child for what you consider to be just naughty behaviour, is probably the worst thing you can do.