I think you are writing about some past entity which has morphed into something different which has only the brand in common with that past. Or how else do you explain: 4min,45sec youtube video 'Louis opens new Macbook Air, immediately loses mind.'
I mean, he doesn't make any genuine effort to actually try to make sense of it. From what I can surmise, the case is presumably "air-tight" except the air ducts along the back edge, and the fan will pull air through the case and over the heatsink. That said, that doesn't necessarily mean it's effective enough, as we can see! hahah :P
You can only dissipate so much heat passively conducted through the surface of the device before the temperature of either the internal components or the surface of the case exceeds what is safe. Getting around that often just means having some amount of forced air to carry heat away. In this case, it's a blower that pulls air in from vents, pulls it over various parts of the cavity (including some amount over that heatsink), and exhausts it out. Given how low the TDP of the CPU in a MacBook Air is, the thermal budget presumably balances.
The CPUs in phones have some sort of TIM (thermal interface material) applied to them, and are either in direct contact with the inner side of the case, or a heatpipe which dissipates the heat elsewhere, over a larger area.
This does nothing of that. It's just sitting there simmering nicely. The only reason i can think of is that they recycled most of a former model, and then shrank the cpu-board which then didn't reach unter the vent anymore.
But how did they even manage to oversee that during testing?
[1] https://youtu.be/iiCBYAP_Sgg
where said Louis discovers placebo cooling.
This is broken by design. They should apply quality control to their minds, because no amount of that elsewhere can fix that.