According to Article 2 of the 14th amendment, those who had after the age of 21 been in open rebellion against the US couldn't be in Congress. George was, even though he was a signatory of the Mississippi Ordinance of Secession and a Confederal Colonel. How did this happen?
I was going to ask a similar question (but about Alexander Stephens) so I'll just post it here.
What was the public's opinion on Alexander Stephens serving in the House in 1873? Also, what was the public's reaction to the pardons by Johnson and the Amnesty act?
/u/flamingblankets has already essentially answered your question. The 1872 Amnesty Act removed most of the voting and office-holding restrictions on former Confederates. Zebulon Vance, the wartime governor of North Carolina, became a senator around the same time, while Joseph Wheeler became a congressman a few years later.