Lasagna that was left out overnight is indeed safe to eat as long as the Lasagna was fully cooked to begin with.
if the raw lasagna was left out overnight then it should not be cooked and eaten.
However if you left cooked lasagna out overnight accidentally then it can be safe to eat it.
You can reheat the lasagna in the oven and the temperatures of the oven should kill any bacteria that could be in the lasagna.
Now if the lasagna was left out longer than one night then no it would not be safe to eat.
But overnight is not long enough for much bacteria to grow in the food.
The USDA has guidelines for food establishments and if a food establishment left lasagna out overnight or even a few hours then they would have to throw it out.
They have to do so because of health regulations.
But at home it's okay to eat left out lasagna.
I've eaten lasagna that was left out overnight before and never gotten sick from it.
Overnight a few bacteria or mold spores may land on the surface of the lasagna but they wouldn't have enough time to grow into a significant amount in a few hours or overnight.