Who could explain what the following means? Since nothing is supposed to leave the device in the first place.
Watching-only wallets and hardware wallets can be encrypted.
It isn't that "nothing" leaves the hardware wallet... it's that
PRIVATE KEYS never leave the hardware wallet...
When you use your HW (Trezor or Ledger Nano S etc) with Electrum, Electrum will read the "xpub" from the device and create a watching only wallet on your PC. Prior to this update, there was no way to password protect this wallet file (or ANY watching-only wallet). Thus anyone could copy your wallet file (or start Electrum on your PC) and, while they could not spend your coins, they could see the complete contents, addresses and transaction history etc.
Now, you can encrypt the entire wallet file on disk (like standard wallet files), so that it cannot be opened and viewed without the device connected. In my opinion, this is a handy feature!