Yes the block has some corrosion and usually the first place to see it in CPU blocks because it is where water flow is lowest. I'm not saying that's what caused the pump to leak, but some people have worried about nickel flaking issues causing metal flakes to damage the pump/bearing. I haven't really seen much on that, but regardless, the water alone (without blocker) does appear to have started some corrosion in the CPU block.
In GPU blocks it usually starts where acrylic sits on top of channels for this same reason.
While nickel does look nice and prevents fingerprints from oxidizing copper, it is prone to corrosion when you don't use a corrosion blocker.
Here you can see the corrosion potential. Small but there between copper and nickel.
Plating is very thin and in direct metal to metal contact with copper. it only takes a microscopic pinhole for water to begin reacting. Then it works it's way under the plating because the water becomes very conductive "electrolite" between the two and the copper is what corrodes due to it's higher index. It eats under the nickel then you have flakes of nickel coming off in bigger pieces. This is why Zinc is considered a sacrificial anode..very high index.
In the end, all metals will corrode. How soon it becomes visible depends on direct metal to metal contact, how much galvanic potial there is, how thick the materials/plating are, and how conductive the fluid is.
A corrosion blocker just slows that process way down. Anyhow, not saying that's what caused the leak to start, but the little bit of corrosion does support why they recommend using their coolant which includes not only a biocide, but also a corrosion blocker. When you mix your own water with biocide, you omit the corrosion blocker.
I think the issue is more that the pump top wasn't tightened correctly or worked loose, or the o-ring hardened/shrunk some how. We just don't see this sort of leak issue very often. The pumps are typically pressure tested at higher than normal operating pressure, so any leak that is present at the factory would fail the QC pressure test.
I'm not sure what it is, just wanted to emphasize that water alone isn't perfectly safe as many like to convey, particularly with plated products..there is risk in running your own brew without a blocker.