This is exactly what I said "I understand all lost BTC are in blockchain, they are not "lost", but people probably forgot them or lost the private keys to access"
people define "lost coins" in different ways, some of which are even weird in my opinion like what DdmrDdmr quoted above about "coins that are not moved"!!!
the only coins that are truly lost are those that are provably unspendable. these coins include
* coins sent to burn addresses like 1BitcoinEaterAddressDontSend459kuE
* coins sent to unspendable output scripts such as OP_RETURN
* newly generated coins that are not claimed by the miners (ie. a block reward that is not claimed in coinbase or claimed smaller than it should be).
(there may be more but i can't think of them right now).
anything else is speculation including coins that people lose private key to.