The case for a continent free designation is easiest to see in astrology itself, which is the subject this entry was created alongside.
The technique in this vocabulary has Mesopotamian foundations, an Egyptian layer, a Greek synthesis, an Indian line of development, a Persian and Arabic systematisation, a Latin European consolidation, and a modern revival drawing on all of them. Filing it under a continent means choosing one of those and demoting the rest. Filing it as global states the actual situation, which is that the thing is the transmission.
The same reasoning applies well beyond astrology. Alchemical practice, divinatory geomancy, the great scriptural religions, calendar systems, and a large part of the herbal and mineral correspondence material this atlas records all have histories that cross continents, often repeatedly and in both directions.
There is a discipline that comes with using the marker. Global must not become the place where an unresearched origin goes to hide. A tradition with a known homeland gets that homeland, and a claim about origin that the sources do not support is recorded as an open question rather than dissolved into a category that makes it disappear. Used carefully, the row records a real property of the material. Used carelessly it would be a way of not knowing something quietly, which is why this entry is written as an instruction to the atlas as much as a description for a reader.