This name is a synonym of Sorbus hupehensis var. hupehensis .
The record derives from RJP
(data supplied on 2011-10-18)
which reports it as a synonym (record 2622) with original publication details: Plantae wilsonianae Plantae wilsonianae; 1911 466 1913.
This name record (from RJP) 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 Sorbus laxiflora in version 1 of The Plant List.