It is like a buffer effect. These are binding sites for cations aka positively charged ions. Every time you water, the cation sites exchange with the water, becoming more similar to the cations in the water.
If you feed, then yes, you could theoretically cause toxicity. If you feed say potassium nitrate, added to RO water, then all the ions in the water you are adding are potassium/ K+ ions. So every time you flush, you exchange the cations in the soil for potassium phosphate. Then you flush whatever is exchanged out of the soil out with the water. Eventually, all your cation sites will contain potassium ions only.
If you have less CEC, this happens faster. If you have more CEC, this happens slower. But high CEC also means it is harder to correct an imbalance in the occupation of cation binding sites.
This is all theory, though. Not sure how easy or hard it is to say flush out all the good ions and get only bad ones. Or how relevant this is to everyday bonsai. I mean, it is relevant. But not sure how you'd specifically match the right CEC substrate with the right fertilizer. I guess it means that the lower number of CEC sites you have, the more careful you need to be that the cations you add to liquid fertilizer contains no deficiencies.
Also, with low CEC, you could get more positive tap water ions sit in your CEC sites, like Na+ or Mg2+, which means things like K+, Ca2+ and NH4+ can't bind there and could be flushed out of the soil.
