Hey, don`t feel bad
I am A 330 driver but for a short time I have flown with A 320, like most.
What I remeber, center tank pumps have a little bit higher pressure than wing pumps, this is so because the center tank fuel is preferred to be used first due to structural reason (i.e wing root has to carry more load if fusalage is heavy). So normally when all pumps are running engines are supplied by center tank pumps. So this means only one pump feeds each engine (because only one pump for each engine in the center tank). This is not a favorable situation at critical phases like take off and landing. Therefore center tanks are automatically forced to be off during take off and landing with slats extension. By this way wing pumps (2 pumps per engine) take over the fuel supply job, and this is saffer.
In your question, if slats are stuck at zero, that means center tank pumps will keep feeding during landing, which is not favorable (only one pump for each engine). That is why, ECAM asks you to switch off center tank pumps to force the main wing pumps to take over fuel supplying job.
I hope I could explain.