Hi,
The sulfur in thionyl chloride is rendered more electrophilic because it is bonded to the highly electronegative Cl and O atoms.
As a result, despite possessing a lone pair, it is pretty electronegative. It also contains good leaving groups in Cl-.
Atom identity is important, but you'd have to compare apples to apples - for example a thiol (RSH) is more nucleophilic than an alcohol (ROH).
Hope this helps!
Katt