thanks for the feedback. note to self, use the SDE command line.
SDETABLE reflects the same info as TOAD. ArcGIS still says different.
I used SDETABLE -o alter_column to make this nullability change to a second feature class. That didn't work any better. As before, the change isn't visible to ArcGIS - ArcCatalog still shows it as not-nullable.
Is there a way to make this sort of change to a feature class such that ArcGIS sees the change, or some way to cause the GDB system tables, such as COLUMN_REGISTRY.OBJECT_FLAGS to refresh or repair?
The alternative seems to be to add a new column, copy over the data from the old column. Seems heavy-handed.