Wraithguard are an elites choice. A wraithlord is just a Heavy Support choice, you can take one whenever you want. They have no link to Wraithguard at all in terms of army selection in a normal Eldar force. You are free to take them just like any other unit.
There is no official shoutcast, there is no need, its clear. To suggest you can't is like saying you can't take marine drednoughts unless you take terminators.
The confusion may have risen because of the Iyaden craftworld sub-list which states you cannot take more wraithlords than you have wraightguard units. But in that list both options count as Troops.
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