This name is a synonym of Symphyotrichum subulatum var. ligulatum (Shinners) S.D.Sundb..
The record derives from TICA
(data supplied on 2012-02-11)
which reports it as a synonym (record BD9C2ACD-2956-473C-B445-0E5DA68FF575).
This name record (from TICA) was not included in version 1 of The Plant List. It is possible the same name was included from a different data source, to check this you can search for Aster exilis in version 1 of The Plant List.