Send as SMS only applies to iMessages (sent over wifi or cell data). You can see this option disppears if you trun off iMessage.
Regular SMS (sent to non-iMessage users) can still be sent. If you send a message in the Messages app and it is green, it is SMS and you will be charge for SMS text message.
I turn off cellular services
Settings > General > Cellular - Cellular data does not turn off all cell use. It turns off the cell data path.
SMS does not use cell data. It uses SMS/text data.
iMessages use cell data, as does web & email.