The record derives from TICA
(data supplied on )
which reports it as a synonym and as an illegitimate name (record
1F1FB547-E4E8-4B22-BE25-95C0540F5B94) with original publication details: "J. Richardson, Bot. App., ed. 2. 1823" 30 1823.
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 Sonchus sibiricus in version 1 of The Plant List.