An LN Hub can place a time lock on any Segwit Infected Blockchain coins.
Even those which which are not part of the hub's channels?
LN Notes are like Bank Notes, completely worthless unless someone is willing to trade you something for it.
There are no LN notes. LN will be transacting BTC.
LN transfers happen Offchain , so their is no verification by the actual blockchain, that what you are trading for even exists, until you cash out.
Verification is very easy for channel participants. It's as easy as checking an address in a blockexplorer today.
Say Bob and Carol have a channel with current funds distribution Bob - 3BTC, Carol - 2BTC. Let's represent this as
Bob-3:2-Carol
Alice wants to pay Carol 1 BTC, but opens a channel with Bob (he plans to build a hub, charges no fee for now), she loads 1 BTC to the channel. So now our small piece of LN looks like:
Alice-1:0-Bob-3:2-Carol
There are 2 channels: Alice-Bob with 1 BTC in it and Bob-Carol with 5 BTC. Bob owns 3 BTC: 0 in his channel with Alice and 3 in his channel with Carol. Alice owns 1 BTC, Carol - 2 BTC.
Now Alice pays 1 BTC to Carol via Bob:
Alice-0:1-Bob-2:3-Carol
As expected, Bob still owns 3 BTC: 1 in his channel with Alice and 2 in his channel with Carol. Alice couldn't pay more than 1 BTC. But now she can use her channel with Bob for receiving up to 1 BTC. For the whole lifetime of a channel, this channel's BTC never leave it.
If now, due to a 51% attack, Alice-Bob channel is no more, this directly affects only Bob, he looses 1 BTC, Alice gains it and can spend again, effectively performing a doublespend, just like with good old onchain transactions. Carol isn't affected, she has all info about her channel with Bob. Bob's problems aren't her problems.
LN are not transacting actual BTC, get that thru your head.
LN Notes are a representation of a BTC or a LTC or whatever segwit infected altcoin blockchain they lock up the coins on.
LN is nothing more than a Locking/Unlocking mechanism which gives 1 or 3 parties the right to move portions of it once the time locks are up.
Example: If those time locks expired due to a hack or network spamming, when you go to redeem your LN note, it will Fail.
Then your so called LN Notes prove to have been worthless.
Offchain Verification can not be trusted,
If they let you check the Onchain BTC address the actual BTC is locked up in, you can use a block explorer to confirm they are onchain.
This would inform you if someone 51% attacked and counterfeit LN, but you have to know the Onchain BTC address.
(Does LN even show the Onchain BTC Address so you can check it?)But if someone evades the time locks so that the BTC can be stolen before you can redeem it, then above check was useless.
It is also useless if the 51% attack occurs after you checked the BTC address.
FYI:
LN security is not secured by the Coin's Blockchain,
LN security is nothing more than a series of complicated Time Locks.