Let's not fixate on plastic. Other materials available as I pointed out. Corals grow on plugs made of wood or concrete. These structures don't have to be plastic and buffering can be accomplished without aragonite rocks. Considering aragonite rocks are man made as well then guessing that can be utilized on these printers yet structure made to suit and made hallow and with joins therefore easier to landscape and sell.
Prefer if the direction was to making it work vs why it won't since reefs already being built this way therefore we know it works.
As I said initially, my complaints were with using plastic, which are the only reef ready options posted here, I've also outlined specific concerns against plastic as well as specific ways of improving designs using conventional plastic.
While 3d printed rock from materials closer to what reefs are actually made of (and aragonite is primarily a naturally occurring material), the practicalities of printing those materials are much more difficult on a size scale that would work for a reef tank. If you've ever seen a 3d printer for ceramics, think in that direction - because of the texture of the material being used, the layers and extrusions have to be much thicker and have to have much less dramatic overhangs to retain their shape. If you're going for a reasonably natural look in a tank, 2-5mm layer heights aren't really going to be it until completely encrusted with coral, and if you're fine with that look, why not just use blocks of ceramic media that are already commercially available and which will have much more porosity for bacteria.
The control of and maybe more importantly huge variety of pore sizes in regular live rock are going to be very challenging to replicate even in an industrial setting, let alone on a home printer setup. You've got to be able to control the mixture getting laid down, and given that 3d printing's requirements for consistency and flow rate are so much more constrained than, for example, casting a block of something, finding good and inexpensive methods to make that porosity is going to be a real challenge that has not yet been solved.
Speaking to live rock as habitat, it is the variety of pore sizes that really gives you the whole benefit. Lets say you have a 3d printable mixture to make aragonite based rocks of shapes you like with reasonable looking size and texture - to give varied size tunnel habitats, line of sight blocking protrusions for small animals, and other minute features that live rock already comes with, you're going to have to again develop technology beyond what exists in terms of fine control of clay like extrusions, dynamic crevice generation so it doesn't have to be manually modeled into the CAD files, or something like injection of something that expands into a gas that generates random pockets in the structure after initially being laid down.
It's not that it could never be done, but I don't think it's something realistically close (honestly, plastic rock is going to be the easiest to engineer those microfeatures because of the much finer nozzle size and layer height and better layer adhesion when first laid down), so while very large sizes already exist, sizes that make sense for a home aquarium and look reasonable are probably still a long way off.
While aside to the other points above, I'm not sure 'but you could add live rock and sand' make a compelling case for plastic 3d printed rock being that viable either. It's not that I think it could never be done, should never be done, or isn't worth talking about - my (and others) bringing up potential issues with its design and implementation is also not some attack on the idea.
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