Can I still go to Simatai at night? Is it open?

Andy, Simatai Great Wall only opens 8:00-18:00 from April to October and 8:00-17:30 from November to March.
You are not allowed to stay there at night, so you may choose to stay in a hostel in the Gubei Water Town nearby which allows overnight stay.

The article on this page says there are opening times at night as well though.

That's true. You can visit the wall at night. It's a good experience. You have to take cable car up and down and the ticket price is CNY160, which can be bought on the spot.

So can I not simply hike the wall in the afternoon and then stay (between tower 5 and 6) at sunset time before descending through cable car? Thanks :)

I'm afraid not. For the visit of daytime, the site will clear tourist at 16:30, and the night visit will start from 19:00-19:30 I remember, for which the tickets will start to be sold at 17:30 and you have to take cable car up and down. If going there recently, you can still catch the sunset on cable car. Good luck!