I am not sure what you are referring to here. If the saddle is in the same position relative to the bottom bracket (ie same height above and same distance behind/possibly in front of the BB) seat tube angle will not affect the saddle to bars distance.
Now if you are saying if a frame will not allow you to achieve the same saddle position relative to the BB, then yes that would be true. But that would be terrible fitting methodology. The height of the saddle and position behind/in front of the BB should be derived from the rider's physiology, preference, type of riding. It should not be determined by the frame itself. If you can not get the saddle to the correct position, you need a new saddle, new seat post, or a new frame that would work. Using the frame to determine what the saddle position should be is backwards.
For your example, why would the rider change the saddle position when moving from the cannondale to the Cervelo? If the saddle position remains the same, then stack and reach is totally usable. If I could not get the saddle in the correct position, I would not buy the Cervelo.
There's no well-justified rule to determine saddle position. The best known is KOPS (knee over pedal spindle), but it is not rooted in any real evidence and it's mostly tradition. Saddle offset is one of the least important fit parameters. Primary parameters are saddle-to-bars distance and drop.
Once you have saddle to bars vector dialed in, you can use your own preferences to move the saddle forward and extend the stem, or vice versa. All it does is shift the center of gravity slightly forward or back, change fore-aft weight distribution, and change the hip angle. High hip angle makes for comfortable pedaling with less muscle strain, but moving too far forward can upset the weight distribution and make it difficult to pedal out of saddle.
Also, I would be careful on using ETT to compare different brands. Because some brands measure ETT differently, so effective comparison may not be possible.
The most straightforward definition of ETT is center-to-center, and it's mathematically equal to reach + stack*tan(90-phi) where phi is seat tube angle. I've checked this for a couple of different brands and results agree with published figures to within 1mm.