 Conditioned entropic Ruzsa distance of a summand

If $U,V,X$ are independent random variables in ${\mathbb{F}}_2^n$, thenIndeed, conditioning reduces entropy and independence giveIn ${\mathbb{F}}_2^n$, $V=U+(U+V)$, so conditional entropy under a deterministic change of variables shows that the left-hand side is unchanged when $U$ and $V$ are exchanged. Averaging the displayed bound with its exchanged version gives the result.