I suspect that in the past not all silicone couplers were equal, i.e., some had rather low melting temperatures and would fail if sufficiently heated without a piece of aluminum in between the coupler and the hot exhaust gasses. I've seen them burn through. Whether the size of the gap made much of a difference is debatable, but it makes sense that aluminum blocking a large section of coupler from contact with the exhaust gasses may give the heat a chance to travel down the aluminum and radiate into space.

Ed Cregger