The article was a little confusing, he was a high ranking member but never the leader. Tirofijo was the leader from the late 60s until 2008. Alfonso Cano led for a few years before he was killed in a raid. He was also a hardliner, peace would have been unlikely.

This is not a super surprising move. The last FARC leader Timochenko is free and the president of the successor political party, I'm sure the US would prefer he and others were also in a US prison instead of Colombian politics. They seem to be doing a good job of reintegrating, at least from the outside. There are dissidents that continue to fight, but in much smaller numbers.