Lower frequencies (like the HF range) can propagate further due to reflection/refraction with the earth and the ionosphere. Increasing the frequency can lead to e-skip and troposhpeheric ducting. But even the HF range has shorter wavelengths than our brains, which operate in much lower frequencies (Hz vs MHz). So you would think that our brainwaves would pass through tin-foil much more easily

But it’s the tin-foil’s electro-conduction that “foils” the electromagnetic waves’ (i.e radio waves’) ability to pass through it. But you would have to have no gaps in the tin-foil to completely block all waves from passing through. So like, an entire foil suit or a walking Faraday cage.

TL;DR - it’s not about the length of the electromagnetic wave, it’s the electro-conduction (insulating) property of the tin-foil that matters