I'm a bit surprised to see CEC cited as a substrate selection criteria. I'm no wizard on this, thus I'm asking the question.

My thought is that the exchange of cations and anions is 100% enabled within a solvent such as water in this case. The ability of a surface to store cations should only really matter in the dry (less than saturated) soil where the surface of the substrates ability to hold a very thin film of water affords the exchange of cations and anions and thus enables the root to produce the reactions required for growth.

Does this make any sense to anyone or am I lost?

IMO this is irrelevant in tanks using EI ferts since they are dosed virtually every day.
But in other tanks the ability of a sub material to absorb/hold/release nutrients is/can
be helpful to the plants it that tank.
When first the ferts are dosed the sub material absorbs some of those nutrients because
they are saturated in the water at that point. Then as the level of nutrients falls because the plants are using them, that part which is stored in the sub material
starts to leach out into the water. This evens out that level more through the week
instead of it being high at the start and gradually tapering to possibly nothing at the end of it. I don't know how/if that may help you, just hope it does.

As a point of reference, a simplified view of saturation is over 250,000 ppm tds(total dissolved solids) or more simply about 25% heavier than distilled water. Most water for human consumption is under 300 ppm tds and our fertilizer additions would do little to change that. So...we're not 'saturating' the water with nutrients....what I think you mean is that we're providing an excess of nutrients when compared to what the plants can absorb or use.

The real issue is that when we put solids into water they dissolve...this means they enter ionic form and are free to combine with a more attractive ion. Think of dry nitrogen fertilizer, KNO3, upon hitting water it become a cation in K+ and an ion, NO3-. This is now aqueous chemistry...the ions are available for reaction within the water and the plants grab what they want.

I guess I'm slowly convincing myself CEC means nothing in an aquarium.