What is it?
They are testing big blocks in another chain or simulator, or what?
this is a testnet for segwit. before a release to public or on the mainnet Core is testing all the improvements.
Do you know how much of it is witness data and how relevant this would be to the kind of usage patterns we have today? Are these kinds of segwit block sizes perhaps more relevant for when LN is launched?
LN is an other story because it is off-chain. just the start and the end is manifested in the blockchain. the other whole lifecycle of certain LN activities does not affect the blockchain.
SegWit is a speration of data from the blockchain which is not immediate necessary for txs. and because of this you can see blocks of 3.6 MB. the data which is necessary is stored in the blockchain and the other data is seperated. for example in sum you have data of 3.6 MB but the necessary data which is stored in the block is just 0.9 MB.
EDIT: I cannot see the exact amount of data which is stored in the block and the exact amout of data whic is excluded from the block
https://segnet.smartbit.com.au/block/00000072e17929d0d4c1a8f3e5f59c4fc10dd4d32861a40da855d55309a97562