There are hardly any existing STEEM tokens on Poloniex because they delisted it a while back. Even then, it was barely used (the delisting was entirely reasonable on low-volume grounds).
Other, more important, exchanges may very well follow suit, but I would agree that Poloniex itself doesn't mean anything here.
Poloniex has actually been staging a bit of a comeback under the new management. But fair enough, as mentioned before, we'll have to wait and see what other exchanges do. As many have pointed out, Binance / CZ is closely allied with Tron / Sun. If Binance supports the swap, that'll pretty much be the confirmation we need.