Cannot get to that ROI.
At ~2usd/day for 61 mhs, I get ~8 months ROI. Without electric cost.
What am I missing?
You aren't missing anything, he is just playing with "house money".
You are correct, his breakeven point on his new cards are about as you've calculated. Even though he has other rigs to pay it off, the breakeven time of the additional capital deployed has to be calculated based on the incremental income it provides.