He figured out segwit in the sense that it didn't need to be a hardfork. In a nutshell the signatures that used to be stored within a block (and count towards the 1 megabyte limit) are now stored "outside" the block so that old nodes simply ignore the signature part of segwit transactions.

One consequence is that pre-segwit nodes consider segwit transaction outputs as "can be spent by anyone" because they don't require a signature.

As long as most people use post segwit nodes, this is no problem of course, but it's funny at least that in theory anyone can create valid transactions to spend literally any and every single segwit UTXO