There's nothing in that final scene suggesting sentimentality or anything else--other than a belief that Genesis might restore spock--particulalry since a conscious choice was made to send Spock's tube there, instead of space. Further, Kirk just witnessed the effect of Genesis literally creating life from a lifeless world, so his log entry reads as acknowledging the technology's potential--that Spock might benefit from the Genesis effect.
He simply assumed that Spock (or rather Spock's body's matter) would be a seed for new life. As they discussed before they were attacked by the Reliant, Genesis would erase existing life and reorganize it (that was Spock's own comment that got McCoy pretty upset).
Had Nimoy never returned to the franchise, Kirk's log entry would still be the same, but Spock would have stayed dead.