Yes, but the idea is that airplane mode can be faked in such a way by software that you cannot be sure its entirely off (remember Android still scanning WLAN/Bluetooth even whilst it was off?). With a hardware kill switch, you have more assurance it is off. With tinfoil, yet even more.
Tinfoil manifests a much worse UX than the kill switch. The foil is physically separate from the phone and can easily get misplaced or torn such that the phone is no longer able to be secured. Moreover, the user can be so physically exhausted that they neglect to retrieve the foil from their fanny pack and properly wrap the phone (not to mention checking for any areas where holes might let the signal escape which takes extra time).
It is at least an order of magnitude quicker for the user to toggle a kill switch that resides at all times on the phone. Negligence due to physical exhaustion is thus eliminated-- as long as the user can physically access the phone they have sufficient energy to click the kill switch. And as long as the user is able to reach the phone they can access the kill switch, which isn't necessarily true for the tin foil.
Additionally, there are a variety of social engineering attacks that can cause a user to become reluctant to wrap their phone in tinfoil, reluctant retrieve the foil, and even reluctant carry their fanny pack with them. None of those attacks exist for clicking a toggle, especially one designed as an elegant part of the phone's facade (which itself may go unnoticed by the would-be attacker).
Finally, toggling an unobtrusive switch on one's phone doesn't invite onlookers to come over and start discussing unwanted topics like PGP keysigning parties, onion sites, and how the average life span would have increased by 20 years if everyone were still using Gopher.
