Lucy refers to Benedict Arnold's horse being shot out from under him at the battle of Saratoga. This actually happened at the battle of Ridgefield.
The gang rides some distance with Arnold to get to Rittenhouse. Then somehow the time machine is nearby when Flynn makes his escape.
Lucy says that the £1,000 offered to Arnold as the incentive to turn traitor would be equivalent to $80,000 in 2016. It would actually be about twice that amount and closer to $175,000.