I have never heard of this but I only have experience with the Nytro. if in fact this is the case it might have something to do with the mounting height of the air box in relationship to the breather line outlet from the tank. I don't think the oil tanks are designed any different internally besides the overall shape to work in the different chassis. the breather line still comes out the top just like the nytro.
The air box in the nytro is above the top of the oil tank and the breather line is shorter in length. So when the Nytro is upside down the air box is below the tank breather outlet so the oil has a short straight shot down the breather line into the air box and out.
In the Apex However, the air box is lower in the chassis so the breather line from the tank to the air box is slightly longer in length. When the sled is upside down the breather line outlet into the air box is level or slightly higher than the tank outlet causing the flow of oil is be more horizontal or slightly uphill to flow out. this would obviously cause the oil to flow more slowly and with the added capacity of the longer line there is a better chance for you to get the sled upright before the oil makes it all the way into the air box its just a general observation so i could be way wrong here Regardless both sleds need a roll over valve to protect against the oil draining out when the sled is upside down.