Let real coefficients $c_\alpha$ be indexed by subsets of an $n$-element set and satisfy $\sum_\alpha c_\alpha^2\leq1$. For $\delta>0$, take the union $S$ of the sets $\alpha$ with $|\alpha|\leq\ell$ and $c_\alpha^2\geq\ell\delta$. Then $|S|\leq1/\delta$.

This is the counting argument for the decoding set in equation (2) of Håstad's paper. Applied to Fourier coefficients using Parseval's identity and $\delta=2^{-\varepsilon s}$, it gives $|S|\leq2^{\varepsilon s}$. The statement below isolates the counting argument; its energy bound is an explicit hypothesis, not an assumed Fourier identity.