Is this was is/was being used on Heed's pirate TDM?
Yes. Walking on vehicles is a very complicated hack if you want acceptable combat while standing on them while also not compromising interpolation for other cases.
For anyone who needs prediction disabled on vehicles, there's two solutions:
The first (and easier) trick is to change the integration for a vehicle's datablock to 0, call ::transmitDatablocks(1) for any client you want seeing it (preferably before a client can interact with it, like when they're done loading datablocks or about to join the game), and set it back. The client won't do integration on it's side then, and if you get control of the vehicle force update functions in the engine on the server, you can disable gravity and do whatever you want with them, i.e. create floating platforms or simple rigid bodies.
The second trick is specific to vehicles that 1. don't have tires and 2. won't use buoyant forces. You need to tell the engine the vehicle is at rest and zero out its linear and angular momentum, and the client won't do integration because it is tricked into thinking it has no forces applied to it. This works great for floating things and also doesn't need to mess with datablocks like the previous solution. But of course the setup is a little more difficult.