In a nutshell, clearances are direct-line thru-the-air distances.  These
would be unaffected by slits (I call them slots) in the PCB.  However,
creepages are along the surface of the PCB and the slots do effectively
increase your creepage distance.  I have often and successfully used slots
for meeting European creepage distance specs, particularly when using 300-mil
pitch optocouplers on hi-voltage circuits.