The point isn't the size of the trade; the point is that smart money might follow Alameda, and so news that Alameda is "shorting" Tether might, on its own, be enough for CZ to ask that Sam stop.

What's not being pointed out here is that to CZ, and actually even to Sam, there really is no distinction here between Alameda and FTX. CZ is calling him on a position taken by Alameda, via their "Exchange chat," to coordinate between FTX and Binance.