You do realize this IS just an insane amount of RAM already!
The problem gets into what the CPU & PCH is able to address as well as the number of pins the SO-DIMM has.
I think you’ll need to look at Intel’s architecture books to see what the limits of the CPU & PCH being used here.
As far as the SO-DIMM this is the limit of direct addressability, but one can architecturally alter the design so instead of two slots, four would be needed then its conceivable you could support 128 GB and using eight slots 256 GB and on and on… But, thats not what this system has! Maybe in 10 years we’ll see the Mini Max ;-} with that level of RAM.
So Yes, this is the hard limit of this Mac Mini.
Frankly, I really can’t see going higher would be useful except the most extreme .25% of Mac users, 64 GB is already only useful to a very small work set - deep graphics/video editors as it is now.
I think if you need that level of RAM you really need to jump to a 2013 Mac Pro or wait till next year when hopefully we’ll see the replacement 2019 Mac Pro.
If I was designing a system going higher than 32 GB I would be using ECC RAM as once you get past 32 GB you do run the risk of environmental corruption of the memory cells. Thats one reason Apple needed to put a Faraday cage around the RAM.