These are perfectly airtight as advertised.
Just be careful with them: when reheating food in the microwave, even with the lid unlocked as instructed, steam will condense on the lid and raise the temperature of the plastic far above 100 degrees, causing it to warp. After that it still seems to seal (maybe), but you will have a wonky lid.
In the freezer, similarly do not lock the lid! As the air cools it will suck the lid inward, and again you have a wonky lid.
I reckon freezing and reheating also can't be good for the rubber seal on the lid.
Rather, just use it in the fridge, and pour the food into a bowl or plate for reheating.