Khalsa and Sikh two separate things?
Guru Gobind codified the Khalsa? which has no relation to Sikh?
Take this for hand :
khu kbIr jn Bey Kwlsy pRym Bgiq ijh jwnI ]4]3]
kahu kabeer jan bheae khaalasae praem bhagath jih jaanee ||4||3||
Says Kabeer, those humble people become pure - they become Khalsa - who know the Lord's loving devotional worship. ||4||3||
quoted from SGGS, for the sake of plamba.
Guru Gobind codified the true sikhs and initiated them into the Khalsa.
The Khalsa and the Sikh are very much inter-related, whether its only stated in the Dasam Granth or not. The quote above is from Bhagat Kabeer, and shows the theory of Khalsa had been around long before Guru Gobind created it.
Khalsa - Brotherhood dedicated to purity of thought and action.
Sikh - seeker of the truth.
When you have reached the position to be able to maintain purity of thought and action, and still continually seek the truth, you become Khalsa.
It can be seen as a goal of a Sikh. As even Kabeer says from the above quote, that the pure ones become the Khalsa.
The Gurus, and many other bhagats simply laid the foundation to become pure, and Guru Gobind added upon this and codified the Khalsa.
As Bhagat Kabeer stated, the Khalsa, the pure, know the Lord's loving devotional worship. Guru Gobind finished the portrait of the Khalsa, and laid the foundation to become similar teachers. As even Guru Gobind, bent down in most humility and talked to Khalsa Panth as if they were his gurus, his teachers, his informers.
Guru Gobind did not randomly create the Khalsa. It was an image prevalent among all the other Gurus and bhagats, Guru Gobind simply finished the portrait, or it can be seen as he finished the goal of a Sikh.
That a Sikh should become part of this pure community, devoted to purity of action and thought, and still continually search for the truth.