Does this mean that it removes the contents of the page?
No. I already told you more than once what happens when checkpoint writes a page. I'm pretty sure I've also told you the definition of 'clean page' more than once, to start with...
They just write the changes back to disk so that the pages are considered clean.
That doesn't say 'clear', 'empty', or 'discard' anywhere.
Please go and read some of the material that I and others have referred you to, Kalen's book especially
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)SQL In The Wild
: Discussions on DB performance with occasional diversions into recoverability
We walk in the dark places no others will enter
We stand on the bridge and no one may pass