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I am not familiar with that standard specifically.  However, usually standards will have some multiple for effective separation distance inside a PCB versus across air.  One standard I remember specifically states that PCB material is worth 6x the distance relative to air.

This method gets around judging how thick a layer there needs to be to qualify as "inside" the PCB.  In the 6x example, if you've got 10 mils of PCB dielectric between two layers (for example), then that's worth the same as 60 mils of air separation.  The rest of the standard then tells you what you're allowed to do with that air separation, and whether that's enough for various purposes.

Read your standard carefully.  There is likely some wording in there like that.