To the best of my knowledge, ventilation holes prevent suffocation, not SIDS. My best advice regarding SIDS is to deliberately ignore it because it is futile to worry as long as the causes remain unknown -- it's called sudden infant death syndrome for a reason.
Also, "air getting trapped in the holes" is a misunderstanding that you can disregard. The fact that there might be trapped air present is not a danger in itself. If you can provide the link to where you read it, we can discuss it in more detail.
Here's my understanding of the importance of a ventilated mattress:
First, a little background. Babies need to develop enough bodily awareness and motor control to react to their environment:
- If you put a newborn down on his belly, he will bury his face in the mattress because does not know how to turn his head to avoid suffocation.
- If you put a stuffed animal in his face, he does not know how to move himself or the toy away to avoid suffocation.
- This is the reason why you should always put newborns to sleep on their back, and why you should keep pillows and blankets and stuffed animals out of a newborn's crib.
Eventually, babies develop the motor skills to turn themselves around, but at first they can only turn themselves face-down. This is still before they learn to turn their head sideways, and this is why the mattress' ventilation capability is important -- to avoid suffocation.
Some time later (weeks?) they learn to turn themselves around again to lie face-up. By this time, the suffocation danger has passed, but as Swati mentions you should still not place the baby face-down.
When you go mattress shopping, you can easily test the ventilation. Just put your face into the mattress and try to inhale -- the mattress passes the test if you can inhale. (Note of course that this test might be gross if the mattress is dusty or dirty.) You can also refer to the mattress' product information, or ask the store staff.