FYI, they make SO-14 versions of these, for just this reason.
You can mill a slot between pins, though quite a small slot is needed to fit between SOIC pins (requiring a small end-mill, PCB will cost slightly more), and as mentioned, this has no effect on clearance, only creepage. If you can apply potting to the area (maybe conformal coating as mentioned, but it needs to be done carefully to get under the package and seal the pins), that will solve that issue, but it's a whole process to go through (in addition, agencies may require an inspection process for approval).
Note that only functional type insulation is required here, and the creepage distance across the semiconductor itself is quite short but they use a compound with low CTI (comparative tracking index). I think the trick is it's loaded with silica, very low resin fraction -- so the plastic is quite hard, and also hardly expands/softens when heated to soldering temperature (obviously quite important!). The resin still turns to char under arcing, but something about it is acceptable, I forget why. Needless to say, any arcing has already destroyed the circuit; they're only concerned about fire hazard and electrical safety.
Tim