Definition of 2-CNF (a.k.a Krom) formula
A unit clause of the form $p$ is the same as the clause $p \lor p$, and so corresponds to the arrow $\lnot p \to p$.

As an example, consider the unsatisfiable CNF
$$ p \land (\lnot p \lor q) \land \lnot q. $$
The implication graph contains the following edges:
$$
\lnot p \to p \\
p \to q,
\lnot q \to \lnot p \\
q \to \lnot q
$$
These can be arranged in a cycle:
$$
\lnot p \to p \to q \to \lnot q \to \lnot p
$$
This cycle contains both $p$ and $\lnot p$ (and also both $q$ and $\lnot q$), showing that the formula is unsatisfiable.