No no no it's not just XI players, it's players who are a sure in or near for their country's XI and on the short list for a World XI.
I'll start with the exceptions and the two that come to mind are Sutcliffe and O'Reilly. Neither are a sure in for their respective country's XI but they would be anywhere else. And neither looks out of place in a ATG XI.
And to illustrate my point better, let's use Eng, Aus and WI.
ATGs (imo):
WI : Viv, Lara, Headley, Sobers, Ambrose, Marshall
Eng: Hobbs, Hutton, Sutcliffe, Hammond,
Barrington (?), Trueman, Barnes, Knott
Aus: Bradman, G Chappell, Smith, Ponting, Lillee, McGrath, O'Reilly, Warne, Gilchrist
That's more than eleven players above but all are imo, atgs. They all are sure ins/nearly surely ins for their native XI and wouldn't look out of place in a world atg XI.