I'm not the best person to answer this question because I've only read about it here and there on this sub so take it with a grain of salt please but what I remember is people theorized that IA will be enacted, people will be arrested. Biden will not be president. That means that Nancy would be acting president. However, Nancy is likely going to be involved in the arrests if IA is acted.

If there is no acting president, then presidency goes to speaker of house. If there is no speaker of the house, it goes to Pompeo.

So this tweet seems to be either straight up them giving up and him stepping down (which I don't personally think is happening), or Pompeo saying that "we won, I know we won, Future Proves Past"

Because he has to be seen as a neutral party. Pence, Pelosi, and Grassley will be arrested, so SoS is next in line.