Yes, the phrase 'giving Uncle Sam his fair share' is a colloquial way of someone saying that they paid their federal taxes. Why the name is Sam specifically, I do not know, but I'm sure the answer is on the internet somewhere.
Viewing post in Episode 6: Plunge, Now Publicly Available comments
You may have more of an idea than I, as I am about to read through Build 6 after having not read anything more recent than Build 3 when it first released around November 2023. I started a fresh playthrough last night, with only vague memories of unrelated plot-points to guide my way. However, I have faith in your powers of observation and inference to discover the truth for yourself.