So, German scientists would have needed 2.5 metric tons of these non-enriched uranium cubes, to create a self-sustaining reaction with a controlled pile of them.
A quantity that was held in possession, and a capability within reach, but divided between two sites, not yet understood, and never realized by German scientists prior to VE day.
Note to the reader: that this would have been for just a pile. A working weapon would have required years more of effort and an enormous investment in industrial plant.
I'm also surprised that you can achieve criticality with any number of cubes of "natural" uranium that hasn't been enriched.
I guess they'd basically be replicating the Oklo reactor at that point? The Oklo ore was originally much richer in U235, by virtue of it being billions of years older than whatever the Germans were messing with.
Edit: thinking about it, I guess Fermi didn't have access to any enriched uranium either, so the use of moderator material must be the trick.
In fact, the Germans thought that heavy water would be the only moderator that would work and this hampered their efforts. Fermi et al. were able to ascertain that a certain purity of graphite could instead be used, since heavy water was difficult to obtain and expensive at the time. By utilising material from a specialty graphite manufacturer they were able to achieve criticality with Chicago Pile-1
The use of a moderator was also the trick at Oklo - an essential part of that natural phenomenon was that the vein of uranium was in a natural aquifer, and so the water served as a natural moderator. (And since it boiled off at higher powers, also served as a natural control mechanism.)
A quantity that was held in possession, and a capability within reach, but divided between two sites, not yet understood, and never realized by German scientists prior to VE day.