Nonsense. They have always closed at 10pm. The main issue is whether you need a Lao visa on arrival; if you do, then you might be prevented from crossing after around 730 or 8pm.
In that case, apply for an e-visa and use that to enter. You'll be allowed to cross until 10pm, though I'd recommend getting there by 9pm if you don't have your own vehicle.
Which is another catch 22 - it could be that there are no buses operating across the bridge after around 8 or 9pm.
I always travel in my own car, so I am not bothered by or interested in the bus schedule but that could be another issue for you. Since you're flying to Chiang Rai, you won't have your own car so you'll be using public transportation.
Therefore, to save yourself any trouble either book an earlier flight (making sure you reach Chiang Rai by 5pm at the very latest) and then catch a cab to the border crossing OR arrive even earlier and use the Chiang Rai-Bokeo bus, which will drop you off in Huay Xai. There are 4 buses a day but the last one leaves quite early so you'll want to arrive in Chiang Rai much earlier in the day.
Otherwise, travel to Chiang Khong, spend the night then cross over the next day.