It's not a heatsink at all it attaches to whatever the cooling system is so surface area doesn't particularly matter in these terms.
Steam Deck SoC is limited to 15W so long as you could hypothetically use 3 of these (they haven't disclosed if these get soaked and are unusable or if you can go over their dissipation) it wouldn't be significantly worse than the fan which is rated at ~2.5W max. However until it's actually tried there's no real way to know.
There is still certainly an efficiency problem which is why I think they've mostly targeted passively cooled devices. Most of their examples have been things like cooling the macbook air, and the only retail product I'm aware of that uses them is a minipc.
To dissipate 15W, you would need 4 of AirJet Minis, 3 would only get you to 12.75W, drawing 4 watts instead of the 2.5W fan at peak. Maybe you could get creative with heat pipes and blowing the exhaust air over extra fins or something, but Idk, that sounds like it's going to be more complicated and take more space than the traditional way - plus I bet even one AirJet Mini will cost a lot more than one fan.
When it comes to passively cooled PCs the size of a laptop, current netbooks can usually keep a 6W celeron/Atom/N100 cool enough with just a basic (finless) copper heat-sink and sometimes a heat pipe, all wrapped up in a sealed plastic case. It seems like it would be very possible to get the passive cooling up to the 8.5 W dissipation of 2 AirJet Minis by just adding a few fins, switching the case to metal, or maybe just cutting a vent in the plastic.
I'm not the apple expert, but Macbook Airs are able to cool around 20W of CPU load passively, right? I'm seeing 36W for an iPad Pro.
Plus, the 2W to run those Air Jets would be a major drain on battery life in laptop built for a 6W CPU.
I mean I'm glad these AirJet people are being inventive and making new things, but right now it just seems like a solution in search of a problem to solve. I hope they find that problem, and solve that problem... But they're going after PCs and phones, neither of which seem like a good fit for the technology right now.