As a 25 year construction worker, many of which have dealt with electricity. Two full years as an electrician.
Running one 240 circuit might sound intimidating. But it's an efficient way to run a lot of juice to one location. All of your large appliances are ran on a 240, if they're electric.
I can't exactly explain to you why, but most efficient electric motors are 240. I agree with his initial recommendation. Without being on site, or having exact measurements, I cannot say that the cfm is uncalled for. I was going off of your 1300sqft est on the attic.
Just know that the things you are looking for are appropriate air exchange. Not too much, not too little.
Edit
With 240, it's more about the amperage. Basically he's talking about still running two 20A circuts, but in one wire. Level two car charges are like 48A 240, and level three is like 100A 240.