In Ocean's 11, the characters are worried that they'll run out of time and Yen will suffocate. But when he eventually leaves the cart he's hiding in, he opens it himself without any trouble.
If Yen can get out of the cart on his own, why would he suffocate?
Even if the cart wasn't delivered to the vault in time, he could still pop out and just get caught.
Some times that they mention the possibility of Yen suffocating:
In Rueben's house the night before:
RUSTY: At 7:30, Yen gets locked in and we're committed. From this point, we have 30 minutes to blow the power or he suffocates.
When they put Yen into the cart:
RUSTY: Amazing! How does that feel? You all right? Want something to read? A magazine?
YEN's hand pops up showing his middle finger.
RUSTY: Alright. Counting down. Thirty minutes of breathing time starts... now.
During the heist, right before they detonate the Pinch:
LIVINGSTON: Basher, we're set.
BASHER: Hang on a minute, chief.
LIVINGSTON: We don't have a minute. Yen's gonna suffocate.