How exactly does replacing DH1 and DH2 with signatures reduce deniability?
$$\begin{aligned}
k &= \text{kdf}(A^{b_e} \| A^{b}_e \| A^{b_e}_e)\\
&= \text{kdf}(B^{a}_e \| B^{a_e} \| B^{a_e}_e)
\end{aligned}$$
Diffie-Hellman can be computed by "either Alice or Bob", however, the question of which computed the shared key is not revealed. If Alice knows Bob's public key, but has not interacted with Bob, she may forge a transcript under Signal's protocol.
$$\begin{aligned}
k &= \text{kdf}(A^{b_e}_e)\\
&= \text{kdf}(B^{a_e}_e)\\
s_A &= \text{sign}_a(A_e)\\
s_B &= \text{sign}_b(B_e)
\end{aligned}$$
Signatures, by definition are not deniable and their mere presence implies some communication occurred. If Alice and Carol exchange private messages and either Carol, or the signal servers, or a MiTM leaks the signed ephemeral $s_A$; then Bob can still forge a transcript. Bob relies on such a leak so he cannot forge a transcript with any user.
Because Bob cannot forge a transcript with just anyone; Alice is going to have a harder time disputing having ever communicated with Bob, although because deniability was only reduced and not broken, she may succeed. This is the only leakage. The transcript remains deniable thanks to Double Ratchet.
$s_B$ is implied by Signal's signed prekey bundles; although it has additional structure so the semi-ephemeral cannot be mistakened as a fresh ephemeral.
On the other hand so long that the average user cannot forge transcripts because the Signal application does not expose such a feature; the deniability can be disputed not only for the liklihood that the handshake occurred, but can "prove" to a degree using forensic analysis that the transcript was not forged.
Worse, many people will blindly accept a screenshot without signatures nor other authenticators or proofs of identity.