How did you determine that the sentence "This proposition is true" returns the value True?

Again, English is messy. Shokwave was noting (and I was acknowledging) that there is the claim of truth.

To me it doesn't seem to return any value. Tordmor correctly notes its truth-state is uncertain.

No he doesn't. He claims it is ambiguous - an entirely different thing. It is an unambiguous claim to be true. Such a claim can itself be false but the meaning is entirely clear. It says it's true!

Contrast with "This statement is false".

These distinctions become relevant when Omega throws you box puzzles like this.