I'm not so quick to say, "give an air cooled engine all the air it can handle regardless of temperature." Air cooled radial airplane engines have cowl flaps that can close to restrict air flow for a reason- to keep the engines warm enough so that piston clearances don't get huge and so that the oil continues to flow.
That said, radial airplane engines have temperature measurements (usually both cylinder head temp and oil temp) to control the cowl flaps and oil cooler doors. Snowblowers don't, so restricting the flow too much will certainly overheat them. But burlap is pretty open material, I doubt he's going to roast the engine with it.
What he's doing is certainly not necessary and probably not very helpful, but its not going to be deadly harmful either.
Story time... I spent a LOT of hours in my youth mowing with my Dad's old K181S powered 1966 JD tractor (which has quietly been sitting in his shed since he retired it in the late 90s and I would love to restore some day, but I digress...). Just a few minutes of mowing with the old Kohler K would result in a layer of grass clippings covering its fixed air intake screen. I would always scrape it off every so often, but it would still cake over with clippings regularly. That engine ran for ~15 years without a hiccup other than eventually turning into an oil burner, at which point we honed the cylinder and re-ringed it and it ran for another 10 years before he parked it (I was married with a kid by then- that was a TOUGH old iron-block engine!).
