Wow, heard this exact story from a neighbor down here.

Man, a garage hitting 100 degrees is exactly where a blanket does almost nothing. The whole point of a water heater blanket is to slow heat loss when the tank sits in a cold space like a basement or crawlspace. If your garage is that hot, the tank is already staying warm on its own and the blanket might even trap too much heat. That $12 a month your neighbor saw could just be from lower hot water use or a mild month, not the blanket. Honestly I would put that $18 toward a timer or just lower the thermostat on the tank instead.

Species matters here as much as grit. Soft woods like pine and poplar burnish and close up fast, so stopping at 180 makes sense. But hard maple is dense and the pores are already tight, so that old guy's advice may not carry over the same way from whatever he built with in the 80s. What stain were you using, oil or water based? Water based dyes behave totally different on sanded vs burnished wood.