"They" don't got anything of the sort. Maybe Odin does.
But, as you recall, Demonreach is sitting atop a major leyline confluence, and it was "simultaneously" constructed in 5 different times all at once.
Pretty sure Ethniu is gonna spot that happening (and trivially avoid it).
The question is more: of the circles that might reasonably have been constructed, what might have been useful?
We don't know who made MacFinn's circle... or where they got the plan. If they could get the plan... and need more than a regular circle - use the big boy.
Would Ethniu notice a circle that had not been empowered? you have to know when to plug it in.