Just to say my opinion from a different starting point, let's the the different full charge V in AGM vs WET batteries.
AGM are full charged at 13 V, while wet ones are at 12,6 V. What does this mean? That if I use a external charger (like CTEK) , where I have 2 different settings, if I try to charge a AGM leaving the setting on "simple" 12V battery, I will charge my battery without problems but NOT totally; not such a great problem indeed. Else, if I set to AGM, I will continue to charge my connected battery risking to "overcharge" it, with the consequences of it, mainly heating and eventually developing gas. Also this in modern chargers is not so risky because the internal processor is monitoring current, and if you give a look to CTEK instructions the AGM setting is also indicated for better in "cold" temperatures * .
So, what's the problem with internal charging systems of hybrid cars? If they are programmed to charge AGM they manage currents for a little higher full charge V than that optimal for wet batteries, so they could continue to send current even in full charged wet battery. Is this a problem? Probably not a big one, as in parallel with battery there are all other 12 V services working , but neither a "optimal" working state.
A possible test could be to read the real V in 12V circuit with car in ready (ON) and decide accordingly to reading if it's possible to use a cheaper wet battery or not: if we read something lower than 13,5 V it could be worth the risk, if 14 , or if you like to live in peace of mind, use the original battery type.