How can I gift Call of Duty 4 to a friend without linking his account?

Solution 1:

No. You and your friend both need to have linked Battle.net accounts to your respective Humble Bundle accounts.

You need to have your account linked as per their help page in order to send the gift.

After you link your Battle.net account to your Humble Bundle account, an option will appear to email your gift to a friend.

From there, your friend just needs to go to their email and click the link.

But after that, your friend will also need to have a Battle.net account linked in order to actually claim the gift.

Solution 2:

Just above the image you linked it clearly states:

Some Activision / Blizzard games will also be giftable from the Download Page. If so you will see this message on the Download Page.

In other words, some games can be gifted only through battle.net and you need to link/create an account to gift them, and others don't require a battle.net account to gift them.