Well, how do you provide suction, human? I'm pretty sure it involves muscles supported on bones (like ribs or jaw) surrounding a body cavity (lungs or mouth) moving the said bones, or pulling on surrounding tissues to increase the volume of that cavity. But yukkuri have no bones.
Perhaps the outer dough/yukkuri skin provides some support, or is able to provide support through some muscle-like action. With that they could suck, and breathe.
But with just that, the pressure in the mouth and pressure of internal bean paste would be correlated, making the presented contraption ineffective.
So, there must be a mechanism for regulating the volume of the mouth cavity, without relying on external support and pressure of surrounding beanpaste. The mouth itself has to be able to "inflate" through some kind of muscle-like action... but it's a very weird action, like a baloon inflating itself without external pressure or support, it also operates on a different principle than muscles, because muscles contract, not expand.
The mouth walls need to have some amazing engineering to effect that.
Why go to such length for physical accuracy? Don't be a sciencefag. We're watching this to have fun, not for building accurate mathematical or biological models.