I'm not sure if this is intentional or a bug but:

After there is one user who claims a nickname, the link on the player profile disappears.
eg. http://www.gametracker.com/player/ANUoldman/8.12.22.106:28930/
However, on the nickname form on /account/manage/, you can still add that profile as a nickname.

Also, once there are more than one users who have claimed the nickname, the link to claim the nickname remains on the page.
eg. http://www.gametracker.com/player/0GRAF1VON3STAUFFENBERG%5E/85.10.198.69:27960/

It is only in the case of one user account linked to one player profile that the player profile does not show the text "Click here to link this player to your profile" it seems... it seems to be unshown by mistake since you can still add it by the other forum. And in other cases you can add multiple users to one nickname.

Updated to always show the "Click here to link this player to your profile."