I don't think there's anything wrong with your child as far as being normal. And, it sounds as though he's ready to begin preschool. He just needs to become accustomed to a different environment that is more structured than what you likely do at home. As far as keeping to himself, he's probably just going to take some time to warm up to the other kids.
As far as not sitting down to eat with the other kids, it might not have been so much a matter of reprimanding, and preschool is a place for children to learn structure, but maybe they were just letting you know it was something that needed to be worked on. Many parents kind of let their kids "graze" or get up and down at mealtimes and they keep things more structured at preschool to prevent all the kids up and roaming instead of eating.
I see in your update that you will be working on some of those things at home and I think that's a great idea. Begin a little structure there first.
Get up, get dressed, sit for breakfast, have some type of play or learning activity, then lunch, etc.
They do get the hang of it and surprisingly, often times, kids are better behaved at preschool because they get used to the structure, know what to expect and really like it.
If you can, have some play dates with kids his own age so he can get some more experience with interacting and I think he'll do fine next time you try the preschool setting.
All little kids have to adjust at some point so I wonder if they were just pointing something out to you as opposed to making you feel bad. It usually takes kids a couple of days, or even a few, to get settled into the new routine.
I wish you the best!