This appears to be fixed in SSMS 18.1. Thank you!
To clarify, the Tools->Options: Text Editor -> Transact-SQL -> Intellisense setting is not persisted or respected. Please see Erland's included screenshot Clipboard01.jpg.
The setting is not persisted. It shuts it off for the current query window, but once you open a new window you have to shut it off again. Also, it is not persisted when closing and re-opening SSMS.
This aligns with our vision for improved manageability, particularly for VLDBs. Will consider for future development.