In Gibbon's day specious seems to have meant attractive or plausible without necessarily meaning false. For example from Chapter XVI of the Decline and Fall:

A reason has been assigned for the conduct of the emperors towards the primitive Christians, which may appear the more specious and probable as it is drawn from the acknowledged genius of Polytheism.

http://www.ccel.org/g/gibbon/decline/volume1/chap16.htm

Bingley

Bingley