What?  Are you being serious?
Jag's was released in 1994, the PC in 1999.  The Jags was a Doom like engine with flat bitmap sprites and with no form of height (stairs, lifts etc).  If anything it is closer to the Alien Trilogy engine.
The PC was a fully 3D engine, 3D models for enemies and your weapons etc.  Rebellion may have transferred techniques they developed during the Jag version to the PC, but no way in hell that's the same engine.  There is just too much difference between them and the development platforms.
Please if you have a source though...