Yeah, the Marlin definition could certainly be clearer, although I’ve seen otherLulzbot printers use the new home location immediately rather than wait for rehoming (there was a thread somewhere on this forum showing a dump from the console). Edit: found it.
In the end I did M206 X3 and then M500 to store the new position (I checked, and previously it had X=0). This also centres my print correctly for the first time. It’s like my printer geometry is off a little.