@Feeder
It's not. Not by the retcon locked box system, and not under the series 1 system this particular SCP operates under. It creates saplings, yes, but the page states they're easily uprooted and shredded safely. it also cannot spawn multiple skeletons, so there's little risk of proliferation unless containement really fucks up. The only element here is the skeleton's intelligence, so its potential to eventually outwit the current procedure of extinguishing him, but it's too hypothetical and remote to justify keter, whether it's by containment difficulty or potential threat.
Here's the thing though, keter in series 1 was purposefully vague. It was essentially an ominous sign at the start of an entry saying "alright, we're done fucking around, this is one SCP you don't want to get complacent about". It was up to the writer to deliver, and this one doesn't really.
This is why locked box was eventually made, to provide a stricter guideline, but it also made it too easy to write a keter.