I can see why magnesium might
have slipped through as you suggest, under the circumstances you describe, but it stretches one's credibility. From experience of being involved in the design, modelling and manufacture of weapons systems, it is certainly true that the ships' design engineers would have included considerations about all
known physical properties of magnesium in the design/build and the performance of the material in operation and under the potential likely field/deployment operational extremes that could be experienced in warfare.
You could put it down to probably being a calculated risk - magnesium does have to get pretty hot to initiate combustion, after all - but then again, incendiary bombs have been a known quantity in warfare for ages - e.g., since early Greek times at least. From memory, I think the 3 vessels that burned up in the Falklands war could not be extinguished and burned till they sank whilst being towed. The same problems have occurred with magnesium-bodied race cars that caught fire in the '60s. Difficult to douse the fire once the metal starts burning - gets up to about 3,000°C.
In all cases, I think the magnesium fire of these structures was not regarded as a cause of death or potential risk to life.
So maybe the magnesium-built vessels were regarded as expendable if they did get hit sufficiently badly so as to catch fire. By that stage they could arguably have fulfilled their design purpose/function anyway, and the asset cost could have been irrelevant.
By the same token that might be true of these downed high tech UAVs (the one in Iran and now one in the Seychelles) - i.e., the military might regard them as being expendable.
Since no-one has a monopoly on defence technology, then for the US to be in the lead, they would need to keep that technology advancing at a relatively rapid rate to keep ahead of the game (i.e., other foreign technology advancements), and so it would presumably only be a matter of time before the artefacts of the existing technology became obsolete anyway. Example is encryption. What once was held to be proprietary and/or "special" military-strategic knowledge becomes commonplace. GPS is another, though I recall that some act of Congress(?) ensured that the publicly accessible GPS does not have the same resolution/"granularity" as the military version.
In any event, as far as these drones go, I think it hardly likely that the US would make the mistake of dropping the equivalent of the Enigma Machine into foreign hands. Thus, saying that they want the UAV debris back, rather than being important, could just be a step in damage control/mitigation. It also would make the foreign power think they have some really valuable intelligence asset, when all they actually have is an expendable and broken drone.
The days of the heavily armored dreadnought battleships are over. The new navies of major world powers will likely consist of two types of submarine, aircraft carriers, missile frigates, and small fast multipurpose attack ships. All will be lightly armored and as cloaked as the state of stealth technology will allow.
That was interesting news to me. I had not realised that this was the case. Perhaps I should have.
I often wondered if Navy vessels were morphing into something else. I suppose the technology itself is kind of "fluid independent" - i.e., will work in air or water.
Land-based technology is probably a different matter. The clever French ensured that there would be no more Maginot lines anyway.