Sure. Stack is the vertical distance from the BB (the center of the pedals) to the top of the frame at the stem. Reach is the horizontal distance forward to the stem. For a given stem length and angle, bikes with a higher stack will have the handlebars higher and more upright.

You can always use stems to make small tweaks, but changing the stem length by more than 20mm can effect handling, and it's hard to find stems with more than 15deg rise. So for example, a 90mm length, 15deg stem will have the handlebars 24mm higher than a 0deg stem.
Lots of bikes are going to higher stack so that people can run a -12deg stem (nearly level) for aesthetics and still achieve a comfortable riding position.

Basically, to match the fit position of a Diverge with no spacers, the Poseidon would need about 30mm of spacers under the stem. If you would ride a Diverge with several spacers under the stem, there's not really a way to get the Poseidon without resorting to very high angle stems. The Poseidon fit looks more like the geometry you'd see in the Pro peloton. I'd be hesitant to recommend it as a starting point for an older rider without a good idea of their fit.
Yes, you can almost certainly make it work, but you may have to resort to things like a 30deg stem or stem extensions.