Pretty much true, it's because there's a serious restriction on the types of electrode which are stable to use with liquid electrolytes. With solid electrolytes we can engineer them to be stable against electrodes with much higher energy densities, i.e. able to hold much more charge. We can also double-up cells with just one metallic electrode in the middle, and then put an electrolyte and an opposite electrode either side so we get 2 battery cells for just 3 electrodes rather than 4, increasing energy density per unit volume. But yeah short of stupidly complex and outrageously unsustainable materials, liquid batteries have definitely reached a plateau in performance. It's good in a way because it gives us the opportunity to create better and safer ones that are way more sustainable than they currently are.

Edit: Forgot to mention that we can also pack solid-state batteries much closer together because overheating isn't an issue, and solid electrolytes actually perform better the hotter they are!