Hole nullity bound: a genuine $u$-hole $H$ with $|H|\le k$ has $|H|<\operatorname{rk}(H)+d$ (Erdős 624)

Setting: $U$ is a finite set of inputs, $L:\mathcal P(U)\to\mathcal P(C)$ an arbitrary list map (the actual colour lists), $k$ a natural number. A colour $u$ is nonbottom if $u\notin L(\varnothing)$ and covering if every $k$-subset $Y\subseteq U$ contains some $A$ with $u\in L(A)$. An actual minimal witness of $u$ is $F\subseteq U$ with $u\in L(F)$ and $u\notin L(F')$ for every $F'\subsetneq F$. A valid label for $u$ is a map $p:U\to V$ into an $\mathbb F_2$-vector space with $\sum_{x\in F}p(x)=0$ for every actual minimal witness $F$ with $1\le|F|\le k$. For $T\subseteq U$, $\operatorname{rk}(T)=\dim\operatorname{span}p(T)$ and $\operatorname{rk}(U)=\dim\operatorname{span}p(U)$ (the dimension of the valid parity space). Theorem. Let $u$ be nonbottom and covering with valid label $p$ of global rank $\operatorname{rk}(U)\ge k-d$ (stated as $k\le\operatorname{rk}(U)+d$). If $H\subseteq U$, $|H|\le k$, is a genuine hole ($u\notin L(A)$ for all $A\subseteq H$), then $|H|<\operatorname{rk}(H)+d$, i.e. the label nullity $\eta(H)=|H|-\operatorname{rk}(H)$ is at most $d-1$.

Source: 'Actual Witness Spans and Intrinsic Kernel Diversity', exact-size hole extension.

Context: Erdős Problem 624 asks whether $H(n)-\log_2 n\to\infty$, where $H(n)$ is the least $m$ such that some $f:\mathcal P(X)\to X$ with $|X|=n$ satisfies $f[\mathcal P(Y)]=X$ for all $|Y|\ge m$. This is an auxiliary finite statement from Sol's Round 13 notes on the two-track (PAIR) case. It does not settle Erdős Problem 624, PAIR or Step 17; the initial near-critical population those notes start from is not established.