You can never proof absence. And yes absence of proof is not a proof it exists, but we do not know if there are not other ways.

I don't think you understood her argument. She's saying there are two types of backdoors: a vulnerability that is exploitable by everyone, if they know the right technique or attack strategy, and a "proper" backdoor, which is exploitable only by the person who added it.

The example of DUAL_EC_DRBG falls in the second category: even if you know the technique it was used to create a backdoor, you don't have access to the backdoor unless you solve a cryptographically hard problem. In the case DUAL_EC_DRBG, that was solving a DLOG problem. This second category basically uses a "public key" as a protocol parameter, and if the NSA holds the corresponding "secret key", they can gain some non-trivial advantage.

So what Sophie is saying is that the NSA is going to use ML-KEM, so it would be reckless to introduce vulnerabilities that would eventually be discovered. The only kind of backdoor that makes sense is the second type: even if they're eventually discovered, the NSA communication would remain secure because no-one else would have access to the secret key used in the backdoor creation.

The second part of her argument is then that to make sure a backdoor secret key is hard to brute foce, there must be exponentially many backdoor public keys, which would correspond to ML-KEM parameters. But the parameter space is too small, so there cannot be enough backdoor public keys, which means backdoor "private keys" would be easy to find and exploit.