Not quite, reading around it seems that flight planning requires you to assume that reverse thrusters and ground spoilers don’t work. So you land, your reverse

Not quite, reading around it seems that flight planning requires you to assume that reverse thrusters and ground spoilers don’t work. So you land, your reverse thrusters immediately fail, and you’re now required to stop using your manual breaks and consume all of the breaking distance you originally planned for, with presumably a bit of margin on top.