Does this mean leaders can ride on tanks? No, because they cannot activate tanks. This gets modified somewhat in late war supplements & games.
So both the truck and the infantry unit are 'moving' simultaneously during the loading? Yes, more or less.
...then the truck carries on moving? Yes.
...then unloads the infantry somewhere else (where it has to stop)? The infantry has to stop, the transport doesn't.
Similarly can an infantry unit move into a truck hex, load and then the truck moves and later unloads the infantry somewhere else in the same phase? Yes, but they would both need to activate in the same segment. That part gets tricky.
Ya know Mark, if you keep this up, you'll start sounding like me.