Do I have to enter Non-covered Securities Code X marked as "provided as a courtesy [...] not provided to the IRS" from my 1099B?

Yes!  You have to report all of your investment sales transactions, not just the ones that are reported to the IRS.

Your transaction is reported to the IRS.

It is the "cost or other basis" amount that is not reported to the IRS.

Without basis, it is considered 100% gain. Not very courteous.

@shinyknight