My problem with that line is that it presumes 3PO was able to pick up on a lot of information about himself, rather than the situation he finds himself in. Previous scenes seem to indicate that people are mostly ignoring him because they find his new state annoying. That line would work well if he had asked somebody like Poe who he was and he said, “All you need to know is that you used to be a highly-trained spy who worked for the Resistance.” Thus, it would be funny when 3PO repeats that same information to a stormtrooper. Alas, we obviously don’t have the resources to produce such a thing.

The line “I am on a mission for the Resistance!” is much more believable since he could glean all of that information from the others. He obviously knows he’s on a mission to rescue somebody called “Chewie”, and we can assume somebody mentioned the Resistance by name off-screen.

Here is a really solid take I got of it, although I have some alternatives: https://drive.google.com/file/d/1skny-Is5lOJW2RHW7v8yuYDc1NwfdJ69/view?usp=sharing