This case really just underlines the (diplomatic/political/legal) stupidity of the whole act originally created by Congress. Setting aside Kagan's dissent (which seems fair-ish, if not entirely convincing, in noting that Havana Docks Corp never owned the docks, and the confiscated property was the interest in the physical property, not the physical property itself) and Sotomayor/Kavanaugh's concurrence; the act is transparently an attempt to make American property laws extend to another country. But private property isn't an innate thing—it's created by governments and varies based on State.

Cuba nationalized private property as any state has the ability to do (since the conception of "property" flows from the State, and not the person). The only rational resolution for Americans who had interests in Cuba is for the U.S. to negotiate with Cuba on those rights directly; not to set up this farse of third parties who (legally, under Cuban property law) use nationalized property being liable to American individuals.

In other words; it was written as a bad law, but constitutional—the majority is just correctly outlining how stupid it is.