Must have heard of him via the Vegapunk research I found I guess? Oda's logic like Robin spending 2 years with Dragon and Sabo and never telling Luffy about it in detail especially. Also what she knows from spending 2 years with the Revolutionaries in general.
Although by that logic, since Ivankov is a right hand/commander of Dragon too, Sanji should have been imparted info by Ivankov too.