This name is a synonym of Yucca gloriosa L..
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 50193950) with original publication details: Rep. (Annual) Missouri Bot. Gard. 13: 80 1902.
This name record (from Tropicos) 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 Yucca flexilis in version 1 of The Plant List.