I noticed the same thing, and filed an issue [1]. The first reply does not fill me with a lot of confidence (but it's unclear to me whether the person is affiliated with the project or not).
I (partially) fixed this issue, and I'm not affiliated in any way with Signal. It's public (https://github.com/signalapp/Signal-TLS-Proxy/pull/2), and it looks like they welcome contributions, because they merged mine.
You'd be building and running these outside of Iran for them to work, which would limit the Iranian government's ability to perform the attack you describe.
[1] https://github.com/signalapp/Signal-TLS-Proxy/issues/6