I think it is assumed. According to the rule on Usurper (10.1e), the area drawn at random must have an imperial unit or you proceed to the next lower sector. The fact that the player is attempting suppression presumes that the area is eligible in the first place. If not, the player would proceed to the next lower sector (towards the galactic access). Since units themselves don't add to suppression, it wasn't recited in the example.
At least that's my interpretation.
But for Independent Empires and Invaders I understand that it is enough with a nexus adjacent to it. yes or not?