The point made by the site is that because it costs £3 to look at each title document, it's not practical to see it at any scale. Sure you can find an individual property's ownership, but you can't search this in any way except by location, and you can't answer questions such as "who is the largest landowner in this area" or "what are the holdings of landowner x" without spending thousands on laboriously finding and buying each title. You can't even see the boundaries of each title unless you have bought the individual maps. You can just see a list of titles within a radius of a point, and then buy those titles to find out their extent. A secret doesn't need to be impossible to find to be closely-guarded, it can just be impractical.
That's not "closely-guarded". What you describe is not active guarding so much a characteristic of the antiquited data-retrieval system and the cost associated with manually serving information from that system.
Cite opposition to attempts to reform that system for a better one (that can be polled programatically for example), and then we can say that calling it "closely guarded" is accurate.
It's similar to calling the data contained in microfilm at your local library "closely-guarded". It's cumbersome to parse, yes. It's kept behind lock and guarded doors (when the library is closed), yes. But access is not reserved for those who meet security and/or idological criteral and vetting. There is no attempt to prevent the decemination of the information to others outside a vetted pool of privilaged users. What makes microfilm difficult to access is not it being closely guarded, but rather just that nobody's gone and digitized it — yet (If this is not true, then just image we're in the year 2007, or 1997). If people trying to digitize such material were hampered by active effort of the keepers of the microfilm because the said "keepers" didn't want the information available more broadly... then that would be "close-guarded". But that is not what's happening.
So yes, it's term is click bait. Or just bad, imprecise writing.
20% of the land records for England do not exist in any form at the Land Registry. I'd say that makes land ownership closely guarded - only the owner knows about it.
It isn't click bait. If you click through to the map and look at London, the owners are almost exclusively public sector or tax-evading shell companies in places like The British Virgin Islands, Panama, Jersey, The Isle Of Man, etc. None of these jurisdictions make company ownership records available, thus they can be considered secretly owned.
Although the registry is technically incomplete, this is not a significant issue.
As registration was introduced (as someone explains above, England is _old_) transfers are obliged to be registered, at first it was transfers for sale, but gradually other transfers were added. It is not possible, since registration was switched on for the last of England, to transfer real property without registering, the transfer is invalid if you try.
So, if the land isn't registered, that means it hasn't been transfered since registration began. You're talking about e.g. miles of empty wilderness that are owned by some old guy somewhere. THAT sort of thing isn't secret, everybody in the area will know who owns that "Oh, that's part of Lord Smith's estate. I expect his son manages it these days, of course the son is 73 so you don't see him about".
> This seems really strange to me. Why should this be the case?
It's not. It's a clickbait title. There is a land registry, and you can look at it to find out who owns land.